!2181 add quant utils

Merge pull request !2181 from chenzhongming/quant_utils

mindspore/train/quant/quant_utils.py

+# Copyright 2020 Huawei Technologies Co., Ltd
+#
+# 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.
+# ============================================================================
+"""quantization utils."""
+
+import numpy as np
+
+
+def cal_quantization_params(input_min,
+                            input_max,
+                            num_bits=8,
+                            symmetric=False,
+                            narrow_range=False):
+    r"""
+    calculate quantization params for scale and zero point.
+
+    Args:
+        input_min (int, list): The dimension of channel or 1.
+        input_max (int, list): The dimension of channel or 1.
+        num_bits (int): Quantization number bit, support 4 and 8bit. Default: 8.
+        symmetric (bool): Quantization algorithm use symmetric or not. Default: False.
+        narrow_range (bool): Quantization algorithm use narrow range or not. Default: False.
+
+    Outputs:
+        scale (int, list): quantization param.
+        zero point (int, list): quantization param.
+
+    Examples:
+        >>> scale, zp = cal_quantization_params([1, 2, 1], [-2, 0, -1], 8, False, False)
+    """
+    input_max = np.maximum(0.0, input_max)
+    input_min = np.minimum(0.0, input_min)
+
+    if input_min.shape != input_max.shape:
+        raise ValueError("input min shape should equal to input max.")
+    if len(input_min.shape) > 1:
+        raise ValueError("input min and max shape should be one dim.")
+    if input_min > input_max:
+        raise ValueError("input_min min should less than input max.")
+    if (input_max == input_min).all():
+        # scale = 1.0, zp = 0.0
+        return np.ones(input_min.shape), np.zeros(input_min.shape)
+
+    if symmetric:
+        quant_min = 0 - 2 ** (num_bits - 1)
+        quant_max = 2 ** (num_bits - 1)
+    else:
+        quant_min = 0
+        quant_max = 2 ** num_bits - 1
+    if narrow_range:
+        quant_min = quant_min + 1
+
+    # calculate scale
+    if symmetric:
+        input_max = np.maximum(-input_min, input_max)
+        input_min = -input_max
+    scale = (input_max - input_min) / (quant_max - quant_min)
+
+    # calculate zero point
+    if symmetric:
+        zp = np.zeros(input_min.shape)
+    else:
+        zp_from_min = quant_min - input_min / scale
+        zp_from_max = quant_max - input_max / scale
+        zp_from_min_error = np.abs(quant_min) + np.abs(input_min / scale)
+        zp_from_max_error = np.abs(quant_max) + np.abs(input_max / scale)
+        zp_double = zp_from_min if zp_from_min_error < zp_from_max_error else zp_from_max
+        if zp_double < quant_min:
+            zp = quant_min
+        elif zp_double > quant_max:
+            zp = quant_max
+        else:
+            zp = np.floor(zp_double + 0.5)
+
+    return scale, zp