Nuke RPC

[carmocca]

What does this PR do?

Removes RPC integration

Everything was found with grep -irn "rpc" * --exclude .git

Arguments:

• Complicated to use at the current state
• Fragile
• Potentially to be replaced by fully sharded
• Can be re-added when there's a demand for it or the technology evolves.
• No public usage found:
  https://github.com/search?q=RPCSequentialPlugin&type=code
  https://www.google.com/search?q=%22RPCSequentialPlugin%22+filetype%3Apy
• Should be re-written to use the pipe implementation in torch 1.9

cc @justusschock @SeanNaren @tchaton @awaelchli @ananthsub

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)
• [n/a] 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)

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

[codecov]

Codecov Report

Merging #8101 (93a3eb5) into master (8bd7b1b) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #8101    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         211     209     -2     
  Lines       13658   13412   -246     
=======================================
- Hits        12512   11799   -713     
- Misses       1146    1613   +467     

[carmocca]

Converting to draft to avoid merge until we know everybody is on the same page