Move some debug examples to the source directory

[carmocca]

What does this PR do?

Moves RandomDataset, MNIST, BoringModel, and BoringDataModule to pytorch_lightning/utilities/debugging_examples.py

Unblocks:

• [PoC] Add KFold - External Loop. #8715 (comment)
• MANIFEST.in and setup.py clean-up #7614 (comment)

See the links for context and reasoning.

The use of these implementations is internal. No backwards compatibility guarantees.

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

None

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[codecov]

Codecov Report

Merging #8776 (5b51377) into master (6f3cb1a) will decrease coverage by 0%.
The diff coverage is 89%.

@@           Coverage Diff           @@
##           master   #8776    +/-   ##
=======================================
- Coverage      92%     92%    -0%     
=======================================
  Files         178     179     +1     
  Lines       14692   14861   +169     
=======================================
+ Hits        13524   13675   +151     
- Misses       1168    1186    +18     

[tchaton]

LGTM !

[Borda]

@carmocca is this still draft?

[carmocca]

@carmocca is this still draft?

No

[carmocca]

Closing because removing the pl_examples directory would be a better solution. These examples currently act as an extension of tests and have very low quality as teaching examples for new users.