MANIFEST.in and setup.py clean-up

[carmocca]

What does this PR do?

• Do not include tests, pl_examples, and benchmarks in the generated wheel (python setup.py bdist_wheel)
• Prune the MANIFEST.in definition for the source distribution (python setup.py sdist)
• Copy the MNIST implementation from our tests into pl_examples
• Removed check-manifest use as it contradicts the output of bdist_wheel:

warning: no previously-included files found matching '.yapfignore'
warning: no previously-included files found matching '*.yaml'
warning: no previously-included files found matching 'Makefile'
warning: no previously-included files matching '*.py' found under directory 'requirements'
no previously-included directories found matching 'docs'
no previously-included directories found matching 'dockers'
no previously-included directories found matching 'legacy'
no previously-included directories found matching 'benchmarks'
no previously-included directories found matching 'notebooks'
no previously-included directories found matching 'pl_examples'
no previously-included directories found matching 'tests'

Credit to @flying-sheep for finding the solution in #7596
Closes #7596
Fixes #10335

Resources:

• https://setuptools.readthedocs.io/en/latest/userguide/package_discovery.html#using-find-or-find-packages
• https://packaging.python.org/guides/using-manifest-in

Manually tested that everything is correctly included/excluded.

Part of https://github.com/PyTorchLightning/internal-dev/issues/132

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?
• [n/a] 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?
• [n/a] 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
• Add labels and milestones (and optionally projects) to the PR so it can be classified

cc @Borda @justusschock @awaelchli @akihironitta

[codecov]

Codecov Report

Merging #7614 (c5281a3) into master (3628c31) will decrease coverage by 0%.
The diff coverage is n/a.

❗ Current head c5281a3 differs from pull request most recent head 692c233. Consider uploading reports for the commit 692c233 to get more accurate results

@@           Coverage Diff            @@
##           master   #7614     +/-   ##
========================================
- Coverage      92%     92%     -0%     
========================================
  Files         217     198     -19     
  Lines       14258   12930   -1328     
========================================
- Hits        13161   11914   -1247     
+ Misses       1097    1016     -81     

[Borda]

pls make the benchmark move to test in separate PR, as it does not have any connection to Manifest/setup cleaning

.