Cast to fp16 before moving to device with deepspeed

[rohitgr7]

What does this PR do?

Blue: master
Orange: PR

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @Borda @tchaton @rohitgr7 @awaelchli @akihironitta @carmocca @justusschock

[carmocca]

IMO this should be part of a bug-fix release

[rohitgr7]

IMO this should be part of a bug-fix release

Then, should I move out the deprecated module from here in a sep PR or just say.. deprecated in 1.7.1 and will be removed in 1.9?

[carmocca]

I would choose the "deprecated in 1.7.1" option, but the other would also be fine.

[codecov]

Codecov Report

Merging #14000 (e97dc32) into master (91dd6a6) will increase coverage by 15%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master   #14000     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         323      323             
  Lines       26319    26366     +47     
=========================================
+ Hits        16107    20126   +4019     
+ Misses      10212     6240   -3972