Add ability for the from_checkpoint style functions to accept a tag or path object.

[jstjohn]

For example:

model.from_pretrained("some_remote_tag")
model.save_pretrained("/my/local/path/")

Also in the CLI it would be good to support either tag based strings or path based strings. If a string happens to match a tag, then we could default to the tag and if it does not we could assume it is a path.

Request from @pstjohn