[deprecate] Deprecate LightningModule.model_size

[roshikouhai]

What does this PR do?

This PR deprecates the model_size property in LightningModule as discussed and agreed in #8343

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

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 update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)
• Did you list all the breaking changes introduced by this pull request?

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• 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
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[pep8speaks]

Hello @roshikouhai! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-20 19:34:34 UTC

[codecov]

Codecov Report

Merging #8494 (075686e) into master (6604fc1) will decrease coverage by 48%.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           master   #8494     +/-   ##
========================================
- Coverage      92%     45%    -48%     
========================================
  Files         217     217             
  Lines       14261   14366    +105     
========================================
- Hits        13164    6428   -6736     
- Misses       1097    7938   +6841