Restructure parsing flow in the LightningCLI

[carmocca]

What does this PR do?

Splitting of some of the changes from #7508

• Breaking changes listed below
• Make seed_everything optional
• Typing fixes
• Docs fixes
• Improve config handling

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

LightningCLI is beta, but these are the non-BC changes

• init_parser now returns the parser instance.
• add_core_arguments_to_parser, parse_arguments now take a parser argument
• instantiate_trainer now takes a config and a list of callbacks.
• Split add_core_arguments_to_parser into add_default_arguments_to_parser + add_core_arguments_to_parser

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
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cc @mauvilsa

[codecov]

Codecov Report

Merging #8721 (f4e5bee) into master (963c267) will decrease coverage by 4%.
The diff coverage is 97%.

@@           Coverage Diff           @@
##           master   #8721    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         169     169            
  Lines       14072   14073     +1     
=======================================
- Hits        13043   12468   -575     
- Misses       1029    1605   +576     

[tchaton]

LGTM !