Remove fp16 restriction in the docstring for DeepSpeedStrategy

[thongonary]

What does this PR do?

You guys added bf16 support to DeepSpeedStrategy but forgot to remove the constraint This is only compatible with precision=16 in the docstring.

This PR will save people like me a few minutes reading the source code to make sure bf16 can be used for DeepSpeedStrategy.

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

Not at all.

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[codecov]

Codecov Report

Merging #13919 (d1cd7ef) into master (7fe5d4b) will decrease coverage by 3%.
The diff coverage is n/a.

❗ Current head d1cd7ef differs from pull request most recent head 1ad6f6e. Consider uploading reports for the commit 1ad6f6e to get more accurate results

@@            Coverage Diff             @@
##           master   #13919      +/-   ##
==========================================
- Coverage      79%      76%      -3%     
==========================================
  Files         103      335     +232     
  Lines        6833    26342   +19509     
==========================================
+ Hits         5425    20111   +14686     
- Misses       1408     6231    +4823