torch.cross' dim default changed to c10::optional instead of int=-1 (#17582)

torch.cross' dim default changed to c10::optional instead of int=-1 (#17582)

Summary:
Argument dim=-1 doesn't work for torch.cross. The signature of the torch.cross has been changed to c10::optional<int64_t> dim instead of int64_t. So based on document "If dim is not given, it defaults to the first dimension found with the size 3." and if dim is specified (even negative) it will use the correspondent dim.

Fixes #17229
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17582

Differential Revision: D14483063

Pulled By: ifedan

fbshipit-source-id: f9699093ec401cb185fd33ca4563c8a46cdcd746