Make `mean` function work across multiple dimensions. (#14252)

Make `mean` function work across multiple dimensions. (#14252)

Summary:
Multi-dimensional `sum` is already implemented, and it's trivial to implement `mean` in terms of `sum`, so just do it.

Bonus: Fix incomplete language in the `torch.sum` documentation which doesn't take into account multiple dimensions when describing `unsqueeze` (at the same time as introducing similar language in `torch.mean`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14252

Differential Revision: D13161157

Pulled By: umanwizard

fbshipit-source-id: c45da692ba83c0ec80815200c5543302128da75c