Replace exit with `sys.exit

[SkafteNicki]

What does this PR do?

Fixes #20666
Fixes #20317

exit() and quit() are only guaranteed to be available in interactive envs, where the site module injects them. Thus, user are sometimes experiencing that they are missing (particular in jupyter notebook/lab). Also from the Python docs they are discourage to be used in production code:
https://docs.python.org/3/library/constants.html#constants-added-by-the-site-module

Use sys.exit instead which should always be available.

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📚 Documentation preview 📚: https://pytorch-lightning--21052.org.readthedocs.build/en/21052/

[bhimrazy]

We also seem to have a few exit() in upgrade_checkpoint.py. Should we replace them as part of this PR as well?

pytorch-lightning/src/lightning/pytorch/utilities/upgrade_checkpoint.py

Line 38 in 6a09f27

exit(1)

pytorch-lightning/src/lightning/pytorch/utilities/upgrade_checkpoint.py

Line 49 in 6a09f27

exit(1)

fyi: @SkafteNicki

[SkafteNicki]

We also seem to have a few exit() in upgrade_checkpoint.py. Should we replace them as part of this PR as well?

pytorch-lightning/src/lightning/pytorch/utilities/upgrade_checkpoint.py

Line 38 in 6a09f27

exit(1)

pytorch-lightning/src/lightning/pytorch/utilities/upgrade_checkpoint.py

Line 49 in 6a09f27

exit(1)

fyi: @SkafteNicki

@bhimrazy good call, fixed now :]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (6a09f27) to head (53775da).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21052   +/-   ##
=======================================
  Coverage      87%      87%           
=======================================
  Files         268      268           
  Lines       23321    23324    +3     
=======================================
+ Hits        20313    20316    +3     
  Misses       3008     3008