Add doc (#6144)

docs/guides/model_convert/convert_from_pytorch/api_difference/ops/torch.index_copy.md

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+## [ 组合替代实现 ]torch.index_copy
+
+### [torch.index_copy](https://pytorch.org/docs/stable/generated/torch.index_copy.html#torch.index_copy)
+
+```python
+torch.index_copy(input, dim, index, source, *, out=None)
+```
+
+Paddle 无此 API，需要组合实现。
+
+### 转写示例
+
+```python
+# Pytorch 写法，dim=0
+y = torch.index_copy(input, 0, index, source)
+
+# Paddle 写法
+y = paddle.scatter(input, index, source)
+
+# Pytorch 写法，dim>0
+y = torch.index_copy(input, dim, index, source)
+
+# Paddle 写法
+times, temp_shape, temp_index = paddle.prod(paddle.to_tensor(input.shape[:dim])), input.shape, index
+input, new_t = input.reshape([-1] + temp_shape[dim+1:]), source.reshape([-1] + temp_shape[dim+1:])
+for i in range(1, times):
+    temp_index= paddle.concat([temp_index, index+len(index)*i])
+y = paddle.scatter(input, temp_index, new_t).reshape(temp_shape)
+```