Allow example to use a revision and work with private models
# What does this PR do?

This PR adds the ability to:
- pick a particular revision for a model checkpoint
- use private models when the user is logged in
with the example scripts.

Just did `run_glue` as a proof of concept, will duplicate to all examples and the new example template if this suits everyone.

+1 on the `model_revision` part

On the `use_auth_token`, I was thinking we could try to implement auto-use of the token _iff_ the model is not public (i.e. send the token for non-existent models and private models, as the server doesn't make a difference for those two cases when unauthorized – you get a 404 in both cases – said differently if you don't have access to a model you shouldn't know whether it's an existing private model)

This will require some implementation changes in file_utils though so it might take a bit of time. 

If you think it's helpful to expose this PR's manual option first, I'm ok with that.<|||||>@LysandreJik Yep totally right! I won't personally get around to adding the feature in file_utils/huggingface_hub in the next 2-3 weeks though, so maybe worth it to merge it like this in the meantime:)<|||||>I think it's important to provide the option right now to let the user play with their private models for those scripts. We can have that flag become `None` later on and default to the right thing when the implementation in `file_utils` permits it then remove it entirely a bit later.<|||||>sounds good<|||||>Sounds good!