XLM-R Tokenizer now passes common tests + Integration tests
XLM-R Tokenizer had a lot of issues that were not identified as no testing was done on it.

closes #2993 
closes #2795
closes #2741
closes #2727
closes #2508

This fixes all the above issues, and works for all official checkpoints as well as other SPM files.

However, there are a few things I dislike about the way things stands, which I'm detailing in the comments below.
This solved a CUDA runtime error for me. Strange! Thanks for this PR!