Support exporting aten::copy_ and aten::index_put to ONNX opset 11 (#26941)
Summary:
- [x] Add more comments and refactor the logic of `ReshapeToAdvancedIndexingFormat`
- [x] Add more description here. Cases that are/aren't supported, and how they are supported.
- [x] Need to merge this PR https://github.com/pytorch/pytorch/issues/27186 to enable testing inplace operators.
We are now supporting exporting aten::copy_ and aten::index_put to ONNX.
Here's a breakdown of the different cases in PyTorch code.
```
# Case 1: Scalar Indices
x[0, 1, 2] = data
# Case 2: Slice Indices
x[1:3, :, ::2] = data
# Case 3: Ellipsis Indices
x[..., 0] = data
# Case 4: Tensor Indices
ind1 = torch.tensor([0, 2])
ind2 = torch.tensor([1, 1])
x[ind1, ind2] = data
# Case 5: Mixing all the above cases
ind1 = torch.tensor([0, 2])
ind2 = torch.tensor([1, 1])
x[1:3, ind1, ind2, ..., 3] = data
```
Limitations:
Tensor indices must be consecutive, and 1-d tensors.
```
# Supported
ind1 = torch.tensor([0, 2])
ind2 = torch.tensor([1, 1])
x[ind1, ind2] = data
# Not supported
ind1 = torch.tensor([0, 2])
ind2 = torch.tensor([1, 1])
ind3 = torch.tensor([[0], [1]])
x[ind1, :, ind2] = data
x[ind3] = data
```
Negative indices are not supported.
```
# Not supported
x[-1] = data
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26941
Differential Revision: D17951030
Pulled By: houseroad
fbshipit-source-id: 4357777072f53aa0bc4b297aa1ee53457a7f8dec
