Update Transformers to huggingface_hub >= 0.1.0
# What does this PR do?

This PR updates the minimum version of huggingface_hub to 0.1.0 and makes all the simplifications related to `HfApi` methods being exposed as functions.

Note that some of the commands that relied on the old HfApi were failing since the switch a few versions ago but no issue was opened. This PR removes the code and just returns a somewhat helpful error message.

This PR also fixes one push_to_hub test for the Trainer, currently failing if there are several GPUs.