Fix pickling of KFoldLoop

[niberger]

What does this PR do?

Make the KFoldLoop example class serializable via pickling.
It allow the use of this example with the ddp_spawn strategy.
Without this fix, the deserialization of KFoldLoop crash by entering an endless recursive call of __getattr__("fitloop").

No test written as this only improve an example.

Fixes #12439

Does your PR introduce any breaking changes? If yes, please list them.

None

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. If you need further help see our docs: https://pytorch-lightning.readthedocs.io/en/latest/generated/CONTRIBUTING.html#pull-request or ask the assistance of a core contributor here or on Slack. Thank you for your contributions.

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[codecov]

Codecov Report

Merging #12441 (5a768c1) into master (d418cf2) will decrease coverage by 3%.
The diff coverage is n/a.

❗ Current head 5a768c1 differs from pull request most recent head 23809c9. Consider uploading reports for the commit 23809c9 to get more accurate results

@@            Coverage Diff            @@
##           master   #12441     +/-   ##
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- Coverage      92%      89%     -3%     
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  Files         207      218     +11     
  Lines       17702    22549   +4847     
=========================================
+ Hits        16235    20081   +3846     
- Misses       1467     2468   +1001     

[krshrimali]

Thanks, @niberger for the contribution! 🚀 LGTM!

[akihironitta]

LGTM!