Fix regression in memory-efficient-fp16 (#1216)
Fix regression in memory-efficient-fp16 (#1216)

Summary:
The fused_adam optimizer divides by the scale while our logic
multiplies by the scale. I'm surprised this even worked. The
first few iterations had nearly similar loss with the old
code and even converged.

However, Jun Ru noticed that the loss are very different after
more iterations.

# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/1216

Reviewed By: myleott, shruti-bh

Differential Revision: D22536377

Pulled By: msbaines

fbshipit-source-id: 9328a1764a1895572c18567f99bee3330f25179e

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq