Added doc for torch.einsum sublist format (#57038)
Summary:
Adds documentation for the new sublist format for `torch.einsum`
closes https://github.com/pytorch/pytorch/issues/21412
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57038
Reviewed By: mruberry
Differential Revision: D28994431
Pulled By: heitorschueroff
fbshipit-source-id: 3dfb154fe6e4c440ac67c2dd92727bb5ecfe289e
