Fix RoBERTa/XLNet Pad Token in run_multiple_choice.py (#3631)

* Fix RoBERTa/XLNet Pad Token in run_multiple_choice.py

`convert_examples_to_fes atures` sets `pad_token=0` by default, which is correct for BERT but incorrect for RoBERTa (`pad_token=1`) and XLNet (`pad_token=5`). I think the other arguments to `convert_examples_to_features` are correct, but it might be helpful if someone checked who is more familiar with this part of the codebase.

* Simplifying change to match recent commits

examples/run_multiple_choice.py

@@ -361,6 +361,7 @@ def load_and_cache_examples(args, task, tokenizer, evaluate=False, test=False):
             args.max_seq_length,
             tokenizer,
             pad_on_left=bool(args.model_type in ["xlnet"]),  # pad on the left for xlnet
+            pad_token=tokenizer.pad_token_id,
             pad_token_segment_id=tokenizer.pad_token_type_id,
         )
         if args.local_rank in [-1, 0]: