Update tensor.view_names / tensor.names_ API (#23973)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23973
Without loss of generality, I describe the API for `tensor.view_names`.
`tensor.names_` has an analogous API.
`tensor.view_names(*names)` returns a view on tensor with named dims `names`.
`names` must be of length `tensor.dim()`; otherwise, if '*' is in `names`,
then it (known as the "glob") is expanded greedily to be equal to the
corresponding names from `tensor.names`.
For example,
```
>>> x = torch.empty(2, 3, 5, 7, names=('N', 'C', 'H', 'W'))
>>> x.view_names('*', 'height', 'width').names
('N', 'C', 'height', 'width')
>>> x.view_names('batch', '*', 'width').names
('batch', 'C', 'H', 'width')
```
tensor.view_names(**rename_map) returns a view on tensor that has
renamed dims as specified in the mapping `rename_map`.
For example,
```
>>> x = torch.empty(2, 3, 5, 7, names=('N', 'C', 'H', 'W'))
>>> x.view_names(W='width', H='height').names
('N', 'C', 'height', 'width')
```
These are different(!!!) from the C++ API, which only allows the
following:
- tensor.view_names(optional<DimnameList>)
C++ API parity for named tensors is not important right now; I am
punting that to the future.
Test Plan: - [namedtensor ci]
Differential Revision: D16710916
Pulled By: zou3519
fbshipit-source-id: 7cb8056c0fb4c97b04c3a2d1dd0f737e0a67ce34
