implement "xy" indexing for torch.meshgrid (#62724)
Summary:
This is step 4/7 of https://github.com/pytorch/pytorch/issues/50276. This allows the use of `"xy"` indexing but doesn't change any defaults.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62724
Reviewed By: heitorschueroff
Differential Revision: D30995290
Pulled By: dagitses
fbshipit-source-id: 08a6a6144b20bc019f68bc3c52e3bbf967976d8f
