Model versioning
**Write-up about the context for this change, and the enabled features is at https://discuss.huggingface.co/t/announcement-model-versioning-upcoming-changes-to-the-model-hub/1914**

In short:

1. changes to the file downloading code used in `from_pretrained()` methods to use the new file URLs backed by huggingface-hosted git repos.
2. changes to the model upload CLI to create a model repo then be able to git clone and git push to it.

LGTM!<|||||>When this PR is merged, how do I modify:
```
        data_cached = cached_path(
            "https://cdn-datasets.huggingface.co/translation/wmt_en_ro-tr40k-va0.5k-te0.5k.tar.gz",
            extract_compressed_file=True,
        )
```
to take advantage of this new API?  https://github.com/huggingface/transformers/blob/master/examples/seq2seq/test_bash_script.py#L27
This code was just merged yesterday.