[fix] Add a cluster environment teardown to clean up environment state

[ananthsub]

What does this PR do?

Discussion in #6853
Fixes part of #6303

https://github.com/PyTorchLightning/pytorch-lightning/blob/f852a4f5925211d6bb9dac231b9185e3e6814b13/pytorch_lightning/plugins/training_type/ddp.py#L283-L285

This is a bug for users who have subsequent calls to

trainer.fit(...)
trainer.test(...)

and who launched with torchelastic. The world size environment variable is mistakenly removed, causing the trainer.test call to fail. As a result, we introduce a teardown hook on the cluster environment to resolve this issue. the teardown for torchelastic and slurm are no-ops, but for the lightning environment, we can remove the world size after the sub-process calls finish.

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[pep8speaks]

Hello @ananthsub! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-15 15:39:47 UTC

[codecov]

Codecov Report

Merging #6942 (f852a4f) into master (b85cfbe) will increase coverage by 0%.
The diff coverage is 92%.

❗ Current head f852a4f differs from pull request most recent head cc6457e. Consider uploading reports for the commit cc6457e to get more accurate results

@@           Coverage Diff           @@
##           master   #6942    +/-   ##
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  Coverage      92%     92%            
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  Files         194     194            
  Lines       12345   12391    +46     
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+ Hits        11320   11422   +102     
+ Misses       1025     969    -56