Not always consider a local model a checkpoint in run_glue
# What does this PR do?

In the `run_glue` script, a local model is automatically considered a checkpoint (which is there to enable users to do --model_path_or_name path_to_specific_checkpoint`) but when using a local model, it can crash if the number of labels is different (cf #10502). This PR fixes that by checking the number of labels inside the potential checkpoint before passing it to the `Trainer`. Another possible fix is to check for the presence of files like `trainer_state.json` (but this only works if the checkpoint was created with a recent version of Transformers).

Fixes #10502 
