standardize CTRL BPE files - upload models to S3
This PR:
- update CTRL BPE files (`vocab.json` and `merges.txt`) to use a single format for sub-word splitting (selected to use `</w>` at the end of words)
- upload CTRL updated vocabulary files and pytorch model (not updated) to AWS.

cc @mfuntowicz 
Also cc @keskarnitish!