fp16_utils.py

#   Copyright (c) 2019 PaddlePaddle Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import logging

import numpy as np

import paddle
from paddle.fluid import core, framework, global_scope
from paddle.fluid.log_helper import get_logger
from paddle.fluid.wrapped_decorator import signature_safe_contextmanager

from .fp16_lists import (
    AutoMixedPrecisionLists,
    black_list,
    get_low_precision_dtypestr,
)

_logger = get_logger(
    __name__, logging.INFO, fmt='%(asctime)s-%(levelname)s: %(message)s'
)

_valid_types = [
    core.VarDesc.VarType.LOD_TENSOR,
    core.VarDesc.VarType.SELECTED_ROWS,
    core.VarDesc.VarType.LOD_TENSOR_ARRAY,
]

_fp16_guard_pattern = "__use_fp16__"


def _rename_arg(op, old_name, new_name):
    """
    If an op has old_name input and output, rename these input
    args new_name.

    Args:
        op (Operator): Current operator.
        old_name (str): The old name of input args.
        new_name (str): The new name of input args.
    """
    op_desc = op.desc
    if isinstance(op_desc, tuple):
        op_desc = op_desc[0]
    op_desc._rename_input(old_name, new_name)
    op_desc._rename_output(old_name, new_name)


def _rename_op_input(program, op_var_rename_map, origin_ops, keep_fp32_ops):
    for block in program.blocks:
        ops = block.ops
        block_id = block.idx
        for op in ops:
            if op not in origin_ops or op in keep_fp32_ops:
                continue
            for name in op.input_arg_names:
                if name in op_var_rename_map[block_id]:
                    op._rename_input(name, op_var_rename_map[block_id][name])


def _dtype_to_str(dtype):
    """
    Convert specific variable type to its corresponding string.

    Args:
        dtype (VarType): Variable type.
    """
    if dtype in [core.VarDesc.VarType.FP16, core.VarDesc.VarType.BF16]:
        # TODO(Xreki): change the returned str to "bf16" for BF16 data type.
        # Currently too many codes use "cast_fp16" as key.
        return 'fp16'
    else:
        return 'fp32'


_keep_layer_norm_scale_bias_to_fp32_flag = True


def _keep_layer_norm_scale_bias_to_fp32(*args):
    global _keep_layer_norm_scale_bias_to_fp32_flag
    if len(args) == 0:
        return _keep_layer_norm_scale_bias_to_fp32_flag
    else:
        assert len(args) == 1 and isinstance(args[0], bool)
        old_value = _keep_layer_norm_scale_bias_to_fp32_flag
        _keep_layer_norm_scale_bias_to_fp32_flag = args[0]
        return old_value


def _keep_fp32_input(op, in_name):
    op_type = op.type
    if op_type == 'batch_norm':
        # Scale, Bias, Mean, Variance should be float32.
        return in_name != 'X'
    if op_type == 'layer_norm' and _keep_layer_norm_scale_bias_to_fp32():
        return in_name != 'X'
    if op_type == 'instance_norm':
        return in_name != 'X'
    if op_type == 'fused_bn_add_activation':
        return in_name not in {'X', 'Z'}
    if op_type == 'resnet_unit':
        return in_name not in {'X', 'FilterX', 'Z', 'FilterZ'}
    if op_type in ['fused_attention', 'fused_feedforward']:
        return in_name in {
            'LnScale',
            'LnBias',
            'Ln2Scale',
            'Ln2Bias',
            "Ln1Scale",
            "Ln1Bias",
        }
    if op_type == 'fused_multi_transformer':
        return in_name in {'LnScale', 'LnBias', 'FFNLnScale', 'FFNLnBias'}
    return False


def _keep_fp32_output(op, out_name):
    op_type = op.type
    if op_type in ['batch_norm', 'fused_bn_add_activation']:
        return out_name != 'Y'
    if op_type == 'layer_norm' and _keep_layer_norm_scale_bias_to_fp32():
        return out_name != 'Y'
    if op_type == 'resnet_unit':
        return out_name not in {'Y', 'ConvX', 'ConvZ'}
    if op_type in ['fused_attention', 'fused_feedforward']:
        return out_name in {
            'LnMean',
            'LnVariance',
            'Ln2Mean',
            'Ln2Variance',
            'Ln1Mean',
            'Ln1Variance',
        }
    return False


def _insert_cast_op(block, op, idx, src_dtype, dest_dtype):
    """
    Insert cast op and rename op's input.

    Args:
        block (Program): The block in which the operator is.
        op (Operator): The operator to insert cast op.
        idx (int): The index of current operator.
        src_dtype (VarType): The input variable dtype of cast op.
        dest_dtype (VarType): The output variable dtype of cast op.

    Returns:
        num_cast_op (int): The number of cast ops that have been inserted.
    """
    num_cast_ops = 0

    for in_name in op.input_names:
        if src_dtype == core.VarDesc.VarType.FP32 and _keep_fp32_input(
            op, in_name
        ):
            continue
        for in_var_name in op.input(in_name):
            in_var = block._find_var_recursive(in_var_name)
            if in_var.type not in _valid_types or in_var.dtype == dest_dtype:
                continue
            # op's input is already casted to dest_dtype before. Set the in_var.name to cast_name.
            cast_name = in_var.name + '.cast_' + _dtype_to_str(dest_dtype)
            casted_var = block._find_var_recursive(cast_name)
            if casted_var and casted_var.dtype == dest_dtype:
                _rename_arg(op, in_var.name, casted_var.name)
                continue

            # insert cast for op's input.
            if in_var.dtype == src_dtype:
                out_var = block.vars.get(cast_name)
                if out_var is None or out_var.dtype != dest_dtype:
                    op_device = op.attr('op_device')
                    # NOTE(wangxi): optimize for pipeline, reduce one send.
                    # if in_var is stop_gradient and prev_op device is `all`,
                    # set cast_op device to `all`, can reduce send cast_var.
                    # TODO: need remove this after we unified the dynamic
                    # and static pipeline interface.
                    if (
                        src_dtype == core.VarDesc.VarType.FP32
                        and in_var.stop_gradient
                    ):
                        prev_op = None
                        if in_var.op is op:
                            prev_op = find_true_prev_op(
                                block.ops, op, in_var_name
                            )
                        elif in_var.op is not None:
                            prev_op = in_var.op

                        prev_op_device = None
                        if prev_op is not None:
                            prev_op_device = prev_op.attr('op_device')

                        if (
                            prev_op_device is not None
                            and 'all' in prev_op_device
                        ):
                            op_device = prev_op_device

                    out_var = block.create_var(
                        name=cast_name,
                        dtype=dest_dtype,
                        persistable=False,
                        stop_gradient=in_var.stop_gradient,
                    )

                    # Only forward program will be inserted cast op, but some ops
                    # has no op_role attr, so here set it direcly. eg. resnet_unit.
                    op_role = (
                        int(core.op_proto_and_checker_maker.OpRole.Forward)
                        if not op.has_attr('op_role')
                        else op.attr('op_role')
                    )
                    block._insert_op_without_sync(
                        idx,
                        type="cast",
                        inputs={"X": in_var},
                        outputs={"Out": out_var},
                        attrs={
                            "in_dtype": in_var.dtype,
                            "out_dtype": out_var.dtype,
                            "op_device": op_device,
                            "op_role": op_role,
                        },
                    )
                    num_cast_ops += 1
                _rename_arg(op, in_var.name, out_var.name)

    for attr_name in ['in_dtype', 'out_dtype', 'dtype']:
        if op.has_attr(attr_name) and is_float_dtype(op.attr(attr_name)):
            op._set_attr(attr_name, dest_dtype)

    return num_cast_ops


def find_true_prev_op(ops, cur_op, var_name):
    """
    Find the true prev op that outputs var_name variable.

    Args:
        ops (list): A list of ops.
        cur_op (Operator): Current operator which has var_name variable.
        var_name (string): Variable name.
    """
    prev_op = []
    for op in ops:
        if op == cur_op:
            break
        for out_name in op.output_names:
            for out_var_name in op.output(out_name):
                if out_var_name == var_name:
                    prev_op.append(op)
    if prev_op:
        if not len(prev_op) == 1:
            raise ValueError(
                "There must be only one previous op "
                f"that outputs {var_name} variable"
            )
        else:
            return prev_op[0]
    return None


def find_true_post_op(ops, cur_op, var_name, search_all=False):
    """
    if there are post ops, return them, if there is no post op,
    return None instead.
    Args:
        ops (list): A list of ops.
        cur_op (Operator): Current operator which has var_name variable.
        var_name (string): Variable name.
        search_all (bool): The type of operator search. Use if \"cur_op\" is not in the \"ops\" set.
    """
    post_op = []
    if search_all:
        """
        \"cur_op\" do not have to be in list of \"ops\". E.g. \"cur_op\" can come
        from startup_prog block and \"ops\" list from main_prog block.
        By setting idx to -1, we'll start looking for post-ops from the top of the list.
        If search_all is False, assume that \"cur_op\" is in \"ops\" list,
        so to reduce the time of search we can start iterating from \"cur_op\" idx.
        """
        idx = -1
    else:
        for idx, op in enumerate(ops):
            if op == cur_op:
                break

    for i in range(idx + 1, len(ops)):
        op = ops[i]
        for in_name in op.input_names:
            for in_var_name in op.input(in_name):
                if in_var_name == var_name:
                    post_op.append(op)

    return post_op


def find_op_index(block_desc, cur_op_desc):
    """ """
    for idx in range(block_desc.op_size()):
        if cur_op_desc == block_desc.op(idx):
            return idx
    return -1


def _is_in_black_varnames(op, amp_lists):
    for in_name in op.input_arg_names:
        if in_name in amp_lists.black_varnames:
            return True

    for out_name in op.output_arg_names:
        if out_name in amp_lists.black_varnames:
            return True

    return False


def _need_keep_fp32(op, unsupported_op_list, use_fp16_guard):
    if op.type in unsupported_op_list:
        # the highest priority condition: If ops don't have fp16 computing kernels,
        # they must be executed in fp32 calculation pattern.
        return True

    # process ops about learning rate
    in_out_arg_names = []
    in_out_arg_names.extend(list(op.input_arg_names))
    in_out_arg_names.extend(list(op.output_arg_names))
    for name in in_out_arg_names:
        if "learning_rate" in name:
            return True

    if use_fp16_guard:
        if op.has_attr("op_namescope") and (
            _fp16_guard_pattern in op.attr("op_namescope")
        ):
            # op in fp16 guard
            return False
        else:
            # op not in fp16 guard
            return True
    else:
        return False


@signature_safe_contextmanager
def fp16_guard():
    """
    As for the pure fp16 training, if users set `use_fp16_guard` to True,
    only those ops created in the context manager `fp16_guard` will be
    transformed as float16 type.

    Examples:
        .. code-block:: python

            import numpy as np
            import paddle
            import paddle.nn.functional as F
            paddle.enable_static()
            data = paddle.static.data(name='X', shape=[None, 1, 28, 28], dtype='float32')
            conv2d = paddle.static.nn.conv2d(input=data, num_filters=6, filter_size=3)

            with paddle.static.amp.fp16_guard():
                bn = paddle.static.nn.batch_norm(input=conv2d, act="relu")
                pool = F.max_pool2d(bn, kernel_size=2, stride=2)
                hidden = paddle.static.nn.fc(pool, size=10)
                loss = paddle.mean(hidden)
    """
    with framework.name_scope(prefix=_fp16_guard_pattern):
        yield


def is_float_dtype(dtype):
    return (
        dtype == core.VarDesc.VarType.FP32
        or dtype == core.VarDesc.VarType.FP16
        or dtype == core.VarDesc.VarType.BF16
        or dtype == core.VarDesc.VarType.FP64
    )


def set_var_dst_dtype(
    op, var_names, block, global_block, dtype, need_set_dtype
):
    low_precison_var_names = set()
    for var_name in var_names:
        var = None
        try:
            var = block._var_recursive(var_name)
        except ValueError as e:
            _logger.debug(f"-- {e}, try to get it in the global block --")
            var = global_block.var(var_name)
            if var is not None:
                _logger.debug(
                    f"-- var {var_name} is got in the global block --"
                )

        if var is None or var.type not in _valid_types:
            continue

        if is_float_dtype(var.dtype):
            low_precison_var_names.add(var_name)
            if need_set_dtype:
                var.desc.set_dtype(dtype)

        _logger.debug(
            "---- op type: {}, var name: {}, var dtype: {} ----".format(
                op.type, var_name, var.dtype
            )
        )

    return low_precison_var_names


def set_param_dtype(program, dtype, amp_lists, use_fp16_guard, level):
    keep_fp32_var_names = set()
    if level == "O1":
        return keep_fp32_var_names
    all_parameters = []
    for block in program.blocks:
        all_parameters.extend(block.all_parameters())
        ops = block.ops
        for op in ops:
            # Currently, lookup_table is in black_list and unsupport_list, it's weight will be
            # set to fp32 in setp 1 of cast_model_tp_fp16. But the weight may be used as matmul's
            # input in transformer, so the weight is also in to_fp16_var_names.
            # TODO(zhangting2020): consider fix auto_parallel_fp16 and remove lookup_table
            # from black_list and unsupport_list.
            if op in ['lookup_table', 'lookup_table_v2']:
                continue
            if _need_keep_fp32(op, amp_lists.unsupported_list, use_fp16_guard):
                for in_name in op.input_names:
                    keep_fp32_var_names = keep_fp32_var_names.union(
                        op.input(in_name)
                    )
            else:
                for in_name in op.input_names:
                    if not core.is_compiled_with_ipu() and _keep_fp32_input(
                        op, in_name
                    ):
                        keep_fp32_var_names = keep_fp32_var_names.union(
                            op.input(in_name)
                        )

    for param in all_parameters:
        if param.name not in keep_fp32_var_names:
            _logger.debug(f"-- set param {param.name} to {dtype} --.")
            param.desc.set_dtype(dtype)
    return keep_fp32_var_names


def op_need_keep_fp32(op, amp_lists, use_fp16_guard):
    need_keep_fp32 = False
    if _need_keep_fp32(
        op,
        amp_lists.unsupported_list,
        use_fp16_guard,
    ):
        need_keep_fp32 = True
    elif amp_lists.black_varnames is not None and _is_in_black_varnames(
        op, amp_lists
    ):
        need_keep_fp32 = True
    elif op.type in amp_lists.black_list:
        need_keep_fp32 = True

    return need_keep_fp32


def get_promote_dtype(op, amp_dtype, block):
    dst_dtype = amp_dtype
    for in_name in op.input_names:
        # for ipu, all inputs must be converted to fp16
        if not core.is_compiled_with_ipu() and _keep_fp32_input(op, in_name):
            _logger.debug(
                "---- Input {} {} should be kept fp32 ----".format(
                    in_name, op.input(in_name)
                )
            )
            continue
        # if this op has inputs
        if in_name:
            for in_var_name in op.input(in_name):
                in_var = block._find_var_recursive(in_var_name)
                if in_var and in_var.dtype == core.VarDesc.VarType.FP32:
                    dst_dtype = core.VarDesc.VarType.FP32
                    break
        else:
            dst_dtype = core.VarDesc.VarType.FP32

    return dst_dtype


def get_amp_dst_dtype(
    op, amp_dtype, level, block, amp_lists, keep_fp32_ops, keep_fp16_ops
):
    if level == 'O2':
        return amp_dtype

    ops = block.ops
    dst_dtype = amp_dtype
    if op.type in amp_lists.gray_list:
        keep_fp32 = False
        keep_fp16 = False
        for in_name in op.input_names:
            # if this op has inputs
            if in_name:
                for in_var_name in op.input(in_name):
                    in_var = block._find_var_recursive(in_var_name)
                    # this in_var isn't the output of other op
                    if in_var.op is None:
                        continue
                    elif in_var.op is op:
                        prev_op = find_true_prev_op(ops, op, in_var_name)
                        if prev_op is None:
                            continue
                    else:
                        prev_op = in_var.op

                    # if it's one of inputs
                    if (
                        prev_op in keep_fp32_ops
                        or prev_op.type in amp_lists.black_list
                    ):
                        dst_dtype = core.VarDesc.VarType.FP32
                    elif (
                        prev_op in keep_fp16_ops
                        or prev_op.type in amp_lists.white_list
                    ):
                        dst_dtype = amp_dtype
    else:
        # For numerical safe, we apply fp32 computation on ops that
        # are not determined which list they should stay.
        dst_dtype = core.VarDesc.VarType.FP32
    return dst_dtype


def process_op_input_and_outputs(op, block, global_block, dtype):
    low_precison_var_names = set()
    # Get the FP16 input because the low_precison_var_names is required for the parameter casting.
    # The dtype of the input is not set to fp16, because it is done in the step 3 of cast_model_to_fp16.
    for in_name in op.input_names:
        # for ipu, all inputs must be converted to fp16
        if not core.is_compiled_with_ipu() and _keep_fp32_input(op, in_name):
            continue
        in_vars = set_var_dst_dtype(
            op,
            op.input(in_name),
            block,
            global_block,
            dtype,
            need_set_dtype=False,
        )
        low_precison_var_names = low_precison_var_names.union(in_vars)
    # Set the output to FP16 because its consumer OP needs to determine if the dtype needs
    # to be promoted.
    for out_name in op.output_names:
        # for ipu, all outputs must be converted to fp16
        if not core.is_compiled_with_ipu() and _keep_fp32_output(op, out_name):
            continue
        set_var_dst_dtype(
            op,
            op.output(out_name),
            block,
            global_block,
            dtype,
            need_set_dtype=True,
        )
    return low_precison_var_names


def cast_model_to_fp16(
    program,
    amp_lists=None,
    use_fp16_guard=True,
    dest_type=core.VarDesc.VarType.FP16,
    level='O2',
    use_promote=False,
):
    """
    Traverse all ops in the whole model and set their inputs and outputs
    to the fp16 data type. This function will do some special process for
    the batch normalization, which keeps the computational process of
    batchnorms in FP32.
    Args:
        program (Program): The used program.
        amp_lists (AutoMixedPrecisionLists): An AutoMixedPrecisionLists object.
        use_fp16_guard(bool): Determine whether to use `fp16_guard` when
                              constructing the program. Default True.
        dest_type(core.VarDesc.VarType): the cast type. such as core.VarDesc.VarType.FP16 and core.VarDesc.VarType.BF16.
    """
    _logger.debug("---- before cast model to fp16 ----")
    _logger.debug(program)
    if amp_lists is None:
        dtype = get_low_precision_dtypestr(dest_type)
        amp_lists = AutoMixedPrecisionLists(dtype)

    # For amp o2 there is no blacklist by default.
    if level == 'O2':
        amp_lists.black_list = amp_lists.black_list - black_list

    global_block = program.global_block()
    keep_fp32_ops = set()
    keep_fp16_ops = set()
    to_fp16_var_names = set()
    keep_fp32_var_names = set()

    # step 1: set params dtype.
    fp32_var_names = set_param_dtype(
        program,
        dtype=dest_type,
        amp_lists=amp_lists,
        use_fp16_guard=use_fp16_guard,
        level=level,
    )
    keep_fp32_var_names = keep_fp32_var_names.union(fp32_var_names)

    def need_process(op):
        need_process = True
        if op.type in ["cast", "create_py_reader", "read"]:
            need_process = False
        else:
            for attr_name in ['out_dtype', 'dtype']:
                if op.has_attr(attr_name) and is_float_dtype(
                    op.attr(attr_name)
                ):
                    need_process = False

        return need_process

    # step 2: divide op into different sets according to the black/unsupported and white lists.
    for block in program.blocks:
        ops = block.ops
        for op in ops:
            _logger.debug(f"-- process op: {op}  --")
            if not need_process(op):
                _logger.debug("---- The op does not need to be processed ----.")
                continue
            if op_need_keep_fp32(op, amp_lists, use_fp16_guard):
                keep_fp32_ops.add(op)
                process_op_input_and_outputs(
                    op, block, global_block, core.VarDesc.VarType.FP32
                )
                _logger.debug(
                    "---- Add into keep_fp32_ops because the op needs to be kept fp32 ----"
                )
            elif op.type in amp_lists.white_list:
                keep_fp16_ops.add(op)
                # get fp16 inputs and set op's outputs to fp16 for promote judgments
                fp16_var_names = process_op_input_and_outputs(
                    op, block, global_block, dest_type
                )
                to_fp16_var_names = to_fp16_var_names.union(fp16_var_names)
                _logger.debug(
                    "---- Add into keep_fp16_ops because the op in white_list ----"
                )
            else:
                # divide others ops into fp16/fp32 sets according to promoting principle.
                dst_dtype = dest_type
                if not use_promote:
                    dst_dtype = get_amp_dst_dtype(
                        op,
                        dest_type,
                        level,
                        block,
                        amp_lists,
                        keep_fp32_ops,
                        keep_fp16_ops,
                    )
                else:
                    dst_dtype = get_promote_dtype(op, dest_type, block)

                if dst_dtype == dest_type:
                    keep_fp16_ops.add(op)
                    fp16_var_names = process_op_input_and_outputs(
                        op, block, global_block, dest_type
                    )
                    to_fp16_var_names = to_fp16_var_names.union(fp16_var_names)
                    _logger.debug(
                        "----  Add into keep_fp16_ops because it should be promoted to fp16 ----"
                    )
                else:
                    keep_fp32_ops.add(op)
                    process_op_input_and_outputs(
                        op, block, global_block, core.VarDesc.VarType.FP32
                    )
                    _logger.debug(
                        "----  Add into keep_fp32_ops because it should be promoted to fp32 ----"
                    )

    # step 3: insert cast op for op's inputs.
    for block in program.blocks:
        ops = block.ops
        idx = 0
        while idx < len(ops):
            op = ops[idx]
            num_cast_ops = 0
            if op in keep_fp16_ops:
                in_var_cast_num = _insert_cast_op(
                    block,
                    op,
                    idx,
                    core.VarDesc.VarType.FP32,
                    dest_type,
                )
                num_cast_ops += in_var_cast_num
            if op in keep_fp32_ops:
                in_var_cast_num = _insert_cast_op(
                    block,
                    op,
                    idx,
                    dest_type,
                    core.VarDesc.VarType.FP32,
                )
                num_cast_ops += in_var_cast_num

            idx += num_cast_ops + 1
    _logger.debug("---- after cast model to fp16 ----")
    _logger.debug(program)

    to_fp16_var_names.difference_update(keep_fp32_var_names)
    return to_fp16_var_names


def _convert_float_to_bfloat16(place, fp32_array):
    paddle.disable_static()
    framework._set_expected_place(place)
    fp32_tensor = paddle.to_tensor(fp32_array)
    bf16_array = paddle.cast(fp32_tensor, paddle.bfloat16).numpy()
    paddle.enable_static()
    return bf16_array


def cast_parameters_to_fp16(
    place,
    program,
    scope=None,
    to_fp16_var_names=None,
    dest_type=core.VarDesc.VarType.FP16,
):
    """
    Traverse all parameters in the whole model and set them to the FP16 data type.
    Whereas, this function will keep parameters of batchnorms in FP32.
    Args:
        place(fluid.CPUPlace|fluid.CUDAPlace): `place` is used to restore the FP16 weight tensors.
        program (Program): The used program.
        scope(fluid.Scope, optional): `scope` is used to get the FP32 weight tensor values.
                                      Default is None.
        to_fp16_var_names(set|list, optional): The data types of vars in `to_fp16_var_names`
                                               will be set to FP16. Usually, it is the returned
                                               value of `cast_model_to_fp16` API.
        dest_type(core.VarDesc.VarType): the cast type. such as core.VarDesc.VarType.FP16 and core.VarDesc.VarType.BF16.
    """
    all_parameters = []
    for block in program.blocks:
        all_parameters.extend(block.all_parameters())

    dtype_str = get_low_precision_dtypestr(dest_type)
    fp16_var_names = to_fp16_var_names if to_fp16_var_names else set()
    var_scope = scope if scope else global_scope()
    for param in all_parameters:
        if param.name in fp16_var_names:
            _logger.debug(
                f"-- cast {param.name} to {dtype_str}, place is {place}"
            )
            if var_scope.find_var(param.name):
                param_t = var_scope.find_var(param.name).get_tensor()
                data = np.array(param_t)
                if dest_type == core.VarDesc.VarType.BF16:
                    bf16_data = _convert_float_to_bfloat16(place, data)
                    param_t.set(bf16_data, place)
                else:
                    param_t.set(np.float16(data), place)
            else:
                _logger.warning(f"Cannot find {param.name}")


def update_role_var_grad(main_prog, params_grads):
    """
    Update op_role_var attr for some ops to make sure the gradients
    transferred across GPUs is FP16.
    1. Check whether the op that outputs gradient is cast or not.
    2. If op is cast and gradient is FP32, remove the op_role_var
       and find the prev op which outputs FP16 gradient
    3. Update the op_role_var of the prev op.

    Args:
        main_prog (Program): The main program for training.
        params_grads (list): A list of params and grads.
    """
    block = main_prog.global_block()
    block._sync_with_cpp()
    BACKWARD = core.op_proto_and_checker_maker.OpRole.Backward
    OPTIMIZE = core.op_proto_and_checker_maker.OpRole.Optimize
    for p, g in params_grads:
        op = g.op
        if g.dtype == core.VarDesc.VarType.FP32 and op.type == 'cast':
            role = op.attr('op_role')
            if role & int(BACKWARD) and op.has_attr('op_role_var'):
                op._remove_attr("op_role_var")
            else:
                raise ValueError(
                    f"The cast op {op} must be in BACKWARD role "
                    "and have op_role_var attr."
                )

            fp16_grad_name = op.input(op.input_names[0])[0]
            op_for_fp16_grad = find_true_prev_op(block.ops, op, fp16_grad_name)
            op_role_var_attr_name = (
                core.op_proto_and_checker_maker.kOpRoleVarAttrName()
            )
            attr_val = [p.name, fp16_grad_name]
            if op_for_fp16_grad.has_attr(op_role_var_attr_name):
                attr_val.extend(op_for_fp16_grad.attr(op_role_var_attr_name))
            op_for_fp16_grad._set_attr(op_role_var_attr_name, attr_val)

            # Maximize the all_reduce overlap, and perform the cast
            # operation after gradients transfer.
            op._set_attr('op_role', OPTIMIZE)
            # optimize op should stay behind forward and backward ops
            if op == block.ops[-1]:
                continue
            post_ops = find_true_post_op(block.ops, op, g.name)
            if post_ops:
                raise ValueError(
                    f"The cast op {op}'s output should not be"
                    "used by a non-optimize op, however, it"
                    f"is used by {post_ops[0]}"
                )
            # add new op in the python and cpp at the same time
            new_op_desc = block.desc.append_op()
            new_op_desc.copy_from(op.desc)
            new_op = framework.Operator(
                block=block,
                desc=new_op_desc,
                type=None,
                inputs=None,
                outputs=None,
                attrs=None,
            )
            block.ops.append(new_op)
            op_idx = find_op_index(block.desc, op.desc)
            if op_idx == -1:
                raise ValueError(f"The op {op} is not in program")
            block._remove_op(op_idx, sync=False)
    block._sync_with_cpp()