refactor and simplify optional imports

[awaelchli]

What does this PR do?

Fixes #2266

refactors code duplication for recurring optional imports like apex, native amp, xla, etc.
it's also backward compatible with e.g. bolts that needs APEX_AVAILABLE etc.

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[codecov]

Codecov Report

Merging #2877 into master will increase coverage by 4%.
The diff coverage is 94%.

@@           Coverage Diff           @@
##           master   #2877    +/-   ##
=======================================
+ Coverage      86%     90%    +4%     
=======================================
  Files          83      80     -3     
  Lines        7861    7185   -676     
=======================================
- Hits         6745    6445   -300     
+ Misses       1116     740   -376     

[Borda]

mind update after #2865

[awaelchli]

@Borda adding this to 0.9.x milestone just in case I have a typo, I don't want it to be released :)

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

• In the file pytorch_lightning/utilities/imports.py:

Line 11:61: W504 line break after binary operator
Line 16:59: W504 line break after binary operator

Comment last updated at 2020-08-24 18:18:41 UTC

[awaelchli]

I see it nowhere used here, so it seems we can easily remove it.

[awaelchli]

removed this, because iterable dataset is part of torch since 1.2 and we support torch >= 1.3