Allow example to use a revision and work with private models

[sgugger]

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.

[julien-c]

+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]

LGTM as is, no opinion regarding the use_auth_token.

Regarding your comment @julien, since fetching a private model with no auth token returns a 404, we should complete the incorrect model identifier error to say that the user may have forgotten its auth token:

OSError: Can't load config for 'lysandre/private'. Make sure that:

- 'lysandre/private' is a correct model identifier listed on 'https://huggingface.co/models'

- or 'lysandre/private' is the correct path to a directory containing a config.json file

If 'lysandre/private' is a private model, make sure your authentication token is correctly setup.

wdyt?

[julien-c]

@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:)

[sgugger]

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.

[julien-c]

sounds good

[LysandreJik]

Sounds good!