Add a main_input_name attribute to all models
# What does this PR do?

This PR adds a new attribute to all models called `main_input_name` which allows us to know whether the model expects `input_ids`, `pixel_values`, `input_values` or `input_features` as a first argument. A test is also added to check the correct value is set for all models, by inspecting the signature.

We could also add some magic property to do what's tested, but I feel an explicit attribute is more customizable for future use cases.