Add ONNX Export Support for torch.scalar_tensor (#28713)
Summary:
Support exporting torch.scalar_tensor() to ONNX.
This will allow making operations on dynamic scalars (like x.size(dim) where x is a tensor of dynamic shape) and exporting them to ONNX.
This is a dummy example of operations that could not be exported dynamically before this PR:
```
size_x = x.size(0)
size_y = y.size(0)
size_x_y_static = torch.tensor([size_x , size_y]) # size_x_y_static is traced as constant
size_x = torch.scalar_tensor(size_x).unsqueeze(0)
size_y = torch.scalar_tensor(size_y).unsqueeze(0)
size_x_y_dynamic = torch.cat((size_x , size_y)) # size_x_y_dynamic is dynamic and depends on x and y's size
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28713
Reviewed By: hl475
Differential Revision: D18438880
Pulled By: houseroad
fbshipit-source-id: c1651e480a41602c7c7452ffc4acba40a2b3827c
