Document default dim for cross being None (#41850)
Summary:
The function torch.cross is a bit confusing, in particular the defaulting of the dim argument.
The default `dim` has been documented as -1 but it is actually `None`. This increases the confusion, in two possible ways depending on how carefully you read the rest. I also add a final warning to the final sentence.
This partially addresses https://github.com/pytorch/pytorch/issues/39310.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41850
Reviewed By: izdeby
Differential Revision: D22664625
Pulled By: albanD
fbshipit-source-id: b8669e026fd01de9e4ec16da1414b9edfaa76bdd
