Add m2m100
# What does this PR do?

Adds the M2M100 model
https://github.com/pytorch/fairseq/tree/master/examples/m2m_100

Fixes #8054
Sure, Patrick !<|||||>I’ve addressed all the review comments, and all the slow/fast tests are now passing.

I didn’t add fast tokenizer because `M2M100` is `sentencepiece`  based tokenizer, but it uses `sentencepiece`  for just tokenizing and then uses a vocab file to convert the tokens to ids and ids to tokens. So our current `SpmConverter` doesn’t work for this. I’ll try to add fast tokenizer in a follow-up PR.

Merging!<|||||>> I’ve addressed all the review comments, and all the slow/fast tests are now passing.
> 
> I didn’t add fast tokenizer because `M2M100` is `sentencepiece` based tokenizer, but it uses `sentencepiece` for just tokenizing and then uses a vocab file to convert the tokens to ids and ids to tokens. So our current `SpmConverter` doesn’t work for this. I’ll try to add fast tokenizer in a follow-up PR.
> 
> Merging!

Hey, I was wondering if there's any progress on a Fast Tokenizer for M2M or if any help can be needed?
Thanks :)