update en document of shard_index_op (#19963)

test=develop
test=document_fix

paddle/fluid/API.spec

@@ -296,7 +296,7 @@ paddle.fluid.layers.deformable_conv (ArgSpec(args=['input', 'offset', 'mask', 'n
 paddle.fluid.layers.unfold (ArgSpec(args=['x', 'kernel_sizes', 'strides', 'paddings', 'dilations', 'name'], varargs=None, keywords=None, defaults=(1, 0, 1, None)), ('document', '3f884662ad443d9ecc2b3734b4f61ad6'))
 paddle.fluid.layers.deformable_roi_pooling (ArgSpec(args=['input', 'rois', 'trans', 'no_trans', 'spatial_scale', 'group_size', 'pooled_height', 'pooled_width', 'part_size', 'sample_per_part', 'trans_std', 'position_sensitive', 'name'], varargs=None, keywords=None, defaults=(False, 1.0, [1, 1], 1, 1, None, 1, 0.1, False, None)), ('document', '99c03e3f249e36854f87dedaa17c8f35'))
 paddle.fluid.layers.filter_by_instag (ArgSpec(args=['ins', 'ins_tag', 'filter_tag', 'is_lod'], varargs=None, keywords=None, defaults=None), ('document', '7703a2088af8de4128b143ff1164ca4a'))
-paddle.fluid.layers.shard_index (ArgSpec(args=['input', 'index_num', 'nshards', 'shard_id', 'ignore_value'], varargs=None, keywords=None, defaults=(-1,)), ('document', '5786fdbba6753ecd6cbce5e6b0889924'))
+paddle.fluid.layers.shard_index (ArgSpec(args=['input', 'index_num', 'nshards', 'shard_id', 'ignore_value'], varargs=None, keywords=None, defaults=(-1,)), ('document', 'c4969dd6bf164f9e6a90414ea4f4e5ad'))
 paddle.fluid.layers.hard_swish (ArgSpec(args=['x', 'threshold', 'scale', 'offset', 'name'], varargs=None, keywords=None, defaults=(6.0, 6.0, 3.0, None)), ('document', '6a5152a7015c62cb8278fc24cb456459'))
 paddle.fluid.layers.mse_loss (ArgSpec(args=['input', 'label'], varargs=None, keywords=None, defaults=None), ('document', 'd9ede6469288636e1b3233b461a165c9'))
 paddle.fluid.layers.data (ArgSpec(args=['name', 'shape', 'append_batch_size', 'dtype', 'lod_level', 'type', 'stop_gradient'], varargs=None, keywords=None, defaults=(True, 'float32', 0, VarType.LOD_TENSOR, True)), ('document', '9d7806e31bdf727c1a23b8782a09b545'))

python/paddle/fluid/layers/nn.py

@@ -14122,52 +14122,46 @@ def deformable_roi_pooling(input,
 
 def shard_index(input, index_num, nshards, shard_id, ignore_value=-1):
     """
-    This layer creates the sharded index for input. This layers is used in
-    model- and data- parallel mixed training generally, in which the index
-    data (usually the label) should be recaculated in each trainer according
-    to 
-
-    .. math::
+    This function recomputes the `input` indices according to the offset of the
+    shard. The length of the indices is evenly divided into N shards, and if
+    the `shard_id` matches the shard with the input index inside, the index is
+    recomputed on the basis of the shard offset, elsewise it is set to
+    `ignore_value`. The detail is as follows:
+    :: 
         
-        assert index_num % nshards == 0
-
-        shard_size = index_num / nshards
-
-        y = x % shard_size if x / shard_size == shard_id else ignore_value
+        shard_size = (index_num + nshards - 1) // nshards
+        y = x % shard_size if x // shard_size == shard_id else ignore_value
 
-    We take the distributed one-hot representation to show what this layer is
-    used for. The distributed one-hot representation is seperated into multiple
-    shards, and each shard is filling zeros except the one with the index
-    inside. In order to create these sharded representation in each trainer,
-    the original index should be recalculated (i.e. sharded) before.
+    NOTE: If the length of indices cannot be evely divided by the shard number,
+    the size of the last shard will be less than the calculated `shard_size`
 
     Examples:
+    ::
     
-        X is a Tensor of integer values:
+        Input:
           X.shape = [4, 1]
           X.data = [[1], [6], [12], [19]]
+          index_num = 20
+          nshards = 2
+          ignore_value = -1
         
-        suppose index_num = 20 and nshards = 2, then we get shard_size = 10
-        
-        if shard_id == 0, we get the Out:
+        if shard_id == 0, we get:
           Out.shape = [4, 1]
           Out.data = [[1], [6], [-1], [-1]]
         
-        if shard_id == 1, we get the Out:
+        if shard_id == 1, we get:
           Out.shape = [4, 1]
           Out.data = [[-1], [-1], [2], [9]]
     
-        the default `ignore_value` -1 is used in this example.
-    
     Args:
-        input(Variable): Input indices, last dimension must be 1.
-        index_num(scalar): An interger defining the range of the index.
-        nshards(scalar): The number of shards
-        shard_id(scalar): The index of the current shard
-        ignore_value(scalar): An ingeter value out of sharded index range
+        - **input** (Variable): Input indices, last dimension must be 1.
+        - **index_num** (scalar): An interger defining the range of the index.
+        - **nshards** (scalar): The number of shards
+        - **shard_id** (scalar): The index of the current shard
+        - **ignore_value** (scalar): An ingeter value out of sharded index range
 
     Returns:
-        Variable: The shard index of input.
+        Variable: The sharded index of input.
 
     Examples:
         .. code-block:: python