addding polynomial lr scheduler (#683)

Summary:
Co-authored-by: jingfeidu <jingfeidu@fb.com>

The implementation is by Jingfei Du from branch "bigbert". Copied over to this CR to get it merged in isolation since other changes seem to be already in master.

**Small changes from original:**
Added following line in `__init__` as discovered by myleott :

```
self.optimizer.set_lr(self.warmup_factor * self.lr)
```
Pull Request resolved: https://github.com/pytorch/fairseq/pull/683

Reviewed By: myleott

Differential Revision: D15149628

Pulled By: myleott

fbshipit-source-id: 5f715611182cdd111e636c66d5f24aa88fa03e29