add tests for the new sharded ddp fairscale integration
This PR adds tests for the just added sharded ddp fairscale integration https://github.com/huggingface/transformers/pull/9139

Obviously these won't run on CIs w/o having fairscale installed... hope we will sort this out down the road. the problem is building fairscale - no binary wheel - I will ask them if they could make a jit version.

@sgugger 
I made a request for a quicker/simpler binary: https://github.com/facebookresearch/fairscale/issues/264

And added an issue to track this so we won't forget: https://github.com/huggingface/transformers/issues/9178
