Fix deepspeed default precision plugin amp_level to O2

[rohitgr7]

What does this PR do?

With

trainer = Trainer(
        strategy=DeepSpeedStrategy(zero_optimization=False),
        amp_backend='apex',
        gpus=2,
        precision=16,
        accelerator='gpu',
)

This error was raised:

RuntimeError: Unexpected optimization level None. Options are 'O0', 'O1', 'O2', 'O3'.  Note that in `O0`, `O1`, etc., the prefix O is the letter O, not the number zero.

But as per docs, default should be O2.

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

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

[codecov]

Codecov Report

Merging #13897 (468c37d) into master (aefb9ab) will increase coverage by 15%.
The diff coverage is 100%.

❗ Current head 468c37d differs from pull request most recent head eb3a02d. Consider uploading reports for the commit eb3a02d to get more accurate results

@@            Coverage Diff            @@
##           master   #13897     +/-   ##
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+ Coverage      61%      76%    +15%     
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  Files         335      333      -2     
  Lines       26301    26163    -138     
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+ Hits        16048    19980   +3932     
+ Misses      10253     6183   -4070