[examples] summarization/bart/finetune.py supports t5
- we were passing attention_mask as an arg, not a kwarg, causing `test_step` to break.
- That case is now covered in the unittest, unittests also cover the t5 model.
- renamed run_bart_sum.py to finetune.py since it is model agnostic. 
- The `bart/` and `t5` subdirectories should be consolidated in a future PR.

This took 15 mins because of underlying infrastructure: unittests for examples and tiny models on S3 :)




Super! LGTM<|||||>Thanks @sshleifer for the quick fix. Just a small query where it will save the output sentences. A file is generated in output_dir with only losses specified when --do_predict is passed in Argument.
What if I want to generate for unknown inputs using fine-tuned model.