Restore signals on teardown

[awaelchli]

What does this PR do?

Restore the default signal handlers on teardown if Lightning configured their own.

Currently, the Trainer / SignalConnector configures signals when running on SLURM or when fault-tolerant training is enabled. These signals are configured in global namespace, and therefore remain intact after Trainer exits. This may not be a problem normally, but one could run into hard to debug issues when running PL in unit tests. This PR makes sure the signals are properly restored.

Found while working on #10601

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @awaelchli @carmocca @justusschock @ninginthecloud @kaushikb11

[codecov]

Codecov Report

Merging #10611 (dc023b1) into master (1437be5) will increase coverage by 48%.
The diff coverage is 100%.

❗ Current head dc023b1 differs from pull request most recent head 3f66a45. Consider uploading reports for the commit 3f66a45 to get more accurate results

@@            Coverage Diff            @@
##           master   #10611     +/-   ##
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+ Coverage      43%      90%    +48%     
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  Files         177      177             
  Lines       16522    16595     +73     
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+ Hits         7035    14986   +7951     
+ Misses       9487     1609   -7878