Fix torch.hub.hub_dir inconsistencies (#38969)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/38401
* `torch.hub.load_state_dict_from_url()` now also downloads to `$TORCH_HOME/hub/checkpoints` instead of `$TORCH_HOME/checkpoints` like `torch.hub.load()` and others.
* Make `hub_dir` private, add and use `get_dir()` instead.
Also updated docs. Did not see a need for additional unit tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38969
Differential Revision: D21725880
Pulled By: ailzhang
fbshipit-source-id: 58cc6b32ddbda91e58c1c1433cc3916223556ea1
