make loggers pickleable

[awaelchli]

What does this PR do?

we need logger pickling in #2512

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[awaelchli]

I'm fixing this logger in #2502 so for now, it will not be picklable.

[codecov]

Codecov Report

Merging #2518 into master will increase coverage by 2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #2518    +/-   ##
=======================================
+ Coverage      88%     90%    +2%     
=======================================
  Files          69      69            
  Lines        5610    5628    +18     
=======================================
+ Hits         4942    5075   +133     
+ Misses        668     553   -115     

[awaelchli]

@williamFalcon these loggers are now picklable

• TensorboardLogger
• CometLogger
• NeptuneLogger (was already)
• TestTubelogger (was already)
  even if we call .experiment on them. when dumping them, we simply remove the _experiment object, which can be recreated from the state of the logger. See the test.

These two are not yet:

• WandbLogger (has problems with pytest, will be fixed in Fix ddp tests + .test() #2512
• MLFlowLogger (has bug with save dir, trying to fix it in Fix ddp tests + .test() #2512)