Feature/torchserve interface [WIP]
Work in progress PR to add a CLI interface for easy packaging of transformers models 
for serving in [pytorch/serve](https://github.com/pytorch/serve). 

Intended usage example:
`transformers-cli torchserve --checkpoint="distilbert-base-uncased-finetuned-sst-2-english" --tokenizer="distilbert-base-uncased" --model-name="distilbert" --task="sentiment-analysis"`

The call above produces a MAR (model archive) file that can be served directly by the `torchserve` binary. 
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
