mBART support for run_summarization.py
Added support for multilingual tokenizer and mBART for run_summarization.py

-> run_summarization.py not working with mBART
So, it appears that this PR introduced a new issue by forcing all models to include `--lang`, @banda-larga would you like to fix it in a new PR and assert on lack of `--lang` only with mbart model type? see: https://github.com/huggingface/transformers/pull/15150#issuecomment-1012553262

and while at it to integrate a better error message as proposed here https://github.com/huggingface/transformers/pull/15150

will then also need to revert this https://github.com/huggingface/transformers/pull/15149 as part of the new PR.

and note to self: make sure 
```
RUN_SLOW=1 pytest tests/deepspeed/test_model_zoo.py::TestDeepSpeedModelZoo::test_zero_to_fp32_zero2_sum_pegasus
```
doesn't fail as this PR broke it. please tag me to the new PR and I will do the checking - you don't need to figure this part out.

Bonus points: adding a new examples test that should have failed with this PR - are we not testing `run_summarization.py` in torch_examples CI?

Thanks.<|||||>This is a tiny bit urgent, so not waiting to remove the two lines that break every existing command using `run_summarization`. I fixed that in [this commit](https://github.com/huggingface/transformers/commit/96881729ce83cfc8e5fa04c903ee4296ad17cfbb).<|||||>ok, so nothing else needs to be done. @banda-larga please ignore my comments above.