Introduce mBart

[kauterry]

What does this PR do? Please describe: Implements the mBart model and its text tokenizer. We are able to successfully load the base model.

Testing the text tokenizer:

VocabularyInfo(size=65539, unk_idx=3, bos_idx=0, eos_idx=2, pad_idx=1)
0 <s>
1 <pad>
2 </s>
3 <unk>
4 .
5 ,
65530 pleasant
65531 ▁glycogen
65532 criminalization
65533 ▁varietal
65534 ▁duplicating
65535 ▁protester
65536 [en]
65537 [es]
65538 <mask>
sample_tokens=tensor([65536,     0,   655,  9692,  2049,    19,    22,   146,    31, 29678,
           13,  1845, 17277,  4120,     5,    56,    22,    15,  5277,     4,
            2], dtype=torch.int32)
decoded_str='Some theories suggest that it may have descendants in Manchuria, but it is unlikely.'
prefix_indices:  tensor([65536,     0])
suffix_indices:  tensor([2])
encoded_tokens=tensor([65536,     0,   655,  9692,  2049,    19,    22,   146,    31, 29678,
           13,  1845, 17277,  4120,     5,    56,    22,    15,  5277,     4,
            2])
round_trip_str='Some theories suggest that it may have descendants in Manchuria, but it is unlikely.'

We see that the encoded_tokens is the same as the sample_tokens and the decoded_str is the same as the round_trip_str.

TODO: Check parity for forward pass through the same checkpoint with fairseq1.

Fixes #{issue number}

Does your PR introduce any breaking changes? If yes, please list them: List of all backwards-incompatible changes.

Check list:

• [ ] Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• [ ] Did you read the contributor guideline?
• [ ] Did you make sure that your PR does only one thing instead of bundling different changes together?
• [ ] Did you make sure to update the documentation with your changes? (if necessary)
• [ ] Did you write any new necessary tests?
• [ ] Did you verify new and existing tests pass locally with your changes?
• [ ] Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)

[kauterry]

@cbalioglu I'm yet to verify parity with the fairseq mBart model by running forward passes. The asset has an internal checkpoint, wondering what the best way to open-source that would be.

[cbalioglu]

@cbalioglu I'm yet to verify parity with the fairseq mBart model by running forward passes. The asset has an internal checkpoint, wondering what the best way to open-source that would be.

You can use one of mBARTs public checkpoints here (e.g. mbart.CC25) to verify parity and include it as an asset card in your PR.