add scatter mapper (#726)

docs/pytorch_project_convertor/API_docs/ops/README.md

@@ -152,7 +152,7 @@
 | 147  | [torch.matmul](https://pytorch.org/docs/stable/generated/torch.matmul.html?highlight=matmul#torch.matmul) | [paddle.matmul](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/matmul_cn.html) |    [差异对比](https://github.com/PaddlePaddle/X2Paddle/tree/develop/docs/pytorch_project_convertor/API_docs/ops/torch.matmul.md) |
 | 148  | [torch.mm](https://pytorch.org/docs/stable/generated/torch.mm.html?highlight=mm#torch.mm) | [paddle.matmul](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/matmul_cn.html) |     [差异对比](https://github.com/PaddlePaddle/X2Paddle/tree/develop/docs/pytorch_project_convertor/API_docs/ops/torch.mm.md)     |
 | 149  | [torch.mv](https://pytorch.org/docs/stable/generated/torch.mv.html?highlight=mv#torch.mv) | 无对应实现                                                   | [组合实现](https://github.com/PaddlePaddle/X2Paddle/tree/develop/docs/pytorch_project_convertor/API_docs/ops/torch.mv.md)  |
-
+| 150  | [torch.scatter](https://pytorch.org/docs/stable/generated/torch.scatter.html?highlight=scatter#torch.scatter) | [paddle.scatter_nd_add](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/scatter_nd_add_cn.html)                                                  | [组合实现](https://github.com/PaddlePaddle/X2Paddle/tree/develop/docs/pytorch_project_convertor/API_docs/ops/torch.scatter.md)  |
 
 
 

docs/pytorch_project_convertor/API_docs/ops/torch.scatter.md

+## torch.scatter
+### [torch.scatter](https://pytorch.org/docs/stable/generated/torch.scatter.html?highlight=scatter#torch.scatter)
+
+```python
+torch.scatter(tensor,
+            dim,
+            index,
+            src)
+```
+
+### [paddle.scatter_nd_add](https://www.paddlepaddle.org.cn/documentation/docs/zh/api/paddle/scatter_nd_add_cn.html)
+
+```python
+paddle.scatter_nd_add(x,
+             index,
+             updates,
+             name=None)
+```
+
+### 参数差异
+| PyTorch       | PaddlePaddle | 备注                                                   |
+| ------------- | ------------ | ------------------------------------------------------ |
+| tensor        | x        | 表示输入Tensor。                                     |
+| dim           | -         | 表示在哪一个维度scatter,Paddle无此参数 |
+| index        | index            | 输入的索引张量                   |
+| src        | updates            | 输入的更新张量                   |
+
+
+
+### 功能差异
+
+#### 使用方式
+因 torch.scatter 与 paddle.scatter_nd_add 差异较大，必须使用 paddle.flatten + paddle.meshgrid + paddle.scatter_nd_add 组合实现，看如下例子
+
+
+### 代码示例
+``` python
+# PyTorch 示例：
+src = torch.arange(1, 11).reshape((2, 5))
+# 输出
+# tensor([[ 1,  2,  3,  4,  5],
+#         [ 6,  7,  8,  9, 10]])
+index = torch.tensor([[0, 1, 2], [0, 1, 4]])
+torch.zeros(3, 5, dtype=src.dtype).scatter_(1, index, src)
+# 输出
+# tensor([[1, 2, 3, 0, 0],
+#         [6, 7, 0, 0, 8],
+#         [0, 0, 0, 0, 0]])
+```
+
+``` python
+# PaddlePaddle 组合实现：
+x = paddle.zeros([3, 5], dtype="int64")
+updates = paddle.arange(1, 11).reshape([2,5])
+# 输出
+# Tensor(shape=[2, 5], dtype=int64, place=CUDAPlace(0), stop_gradient=True,
+#        [[1 , 2 , 3 , 4 , 5 ],
+#         [6 , 7 , 8 , 9 , 10]])
+index = paddle.to_tensor([[0, 1, 2], [0, 1, 4]])
+i, j = index.shape
+grid_x , grid_y = paddle.meshgrid(paddle.arange(i), paddle.arange(j))
+# 若 PyTorch 的 dim 取 0
+# index = paddle.stack([index.flatten(), grid_y.flatten()], axis=1)
+# 若 PyTorch 的 dim 取 1
+index = paddle.stack([grid_x.flatten(), index.flatten()], axis=1)
+# PaddlePaddle updates 的 shape 大小必须与 index 对应
+updates_index = paddle.stack([grid_x.flatten(), grid_y.flatten()], axis=1)
+updates = paddle.gather_nd(updates, index=updates_index)
+paddle.scatter_nd_add(x, index, updates)
+# 输出
+# Tensor(shape=[3, 5], dtype=int64, place=CUDAPlace(0), stop_gradient=True,
+#        [[1, 2, 3, 0, 0],
+#         [6, 7, 0, 0, 8],
+#         [0, 0, 0, 0, 0]])
+```