Disable pl optimizer temporarily to fix AMP issues

[SeanNaren]

What does this PR do?

Temporary hotfix to address issues of #5159.

It seems in this part of the code:

https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/plugins/native_amp.py#L54-L55

PL lightning optimizer results in different outputs. Not entirely sure yet but need to debug and track in the associated issue.

Fixes # (issue)

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[Borda]

btw, there are quite many failing tests...

[SeanNaren]

I've added a todo and a fix (hopefully it builds fine now), seems like there is an underlying internal issue. We wrap the model in the lightning optimizer internally and never unwrap it. If a user specifies that they'd not like to wrap their model in the pl optimizer, not sure why we never unwrap cc @tchaton

[codecov]

Codecov Report

Merging #5163 (e0c9019) into master (81fd33b) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5163   +/-   ##
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  Coverage      93%     93%           
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  Files         134     134           
  Lines        9928    9928           
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  Hits         9242    9242           
  Misses        686     686