Allow `align_to` to take in partially named tensors (#27308)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27308
Currently, `tensor.align_to(*names)` has the restriction that the
`tensor` must be fully named. This doesn't need to be the case, when
using Ellipsis, we "expand the ellipsis to all unmentioned dimensions,
in the order which they appear in the original tensor".
For example, consider `tensor: Tensor[None, None, C]`.
`tensor.align_to(C, None, None)` is ambiguous because the user might
have wanted to switch the order of the None dimensions and there is no
way to specify that using this API. However, `tensor.align_to('C', ...)`
isn't ambiguous: we can select the two unnamed dimensions in the order
in which they appear.
To actually implement this, we write a brand-new `align_to(names,
ellipsis_idx)` function in c++ that is separate from the regular
`align_to(names)` implementation. Ideally we would support "..." as a
special name in c++ and combine the two implementations; we'll need to
support "..." in c++ in the future but that requires a bit of extra work.
In this PR, Python processees the ellipsis and then calls the correct
overload.
Test Plan: - run tests
Differential Revision: D17745179
Pulled By: zou3519
fbshipit-source-id: 9fed06d224215cfb7efecd8c002604baab3c45e6
