repro: parallel execution of stages

[sjawhar]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

TODO

• Tests
• Add --jobs argument to CLI

OTHER THOUGHTS

Obviously, when adding a feature like parallel execution, one of the first objections will be something like "but what if two stages can't run in parallel?". The quick answer is: then don't run in parallel!

But I think it would also be pretty easy to accommodate this in a fairly generic way. For example:

stages:
  foo:
    cmd: python blah.py
    mutex: [foobar]
    # other stage properties

  bar:
    cmd: python other_thing.py
    mutex: [foobar, also_another_one]

Probably self-explanatory, but the idea behind this is:

• No two stages that belong to the same mutex group should be run concurrently
• A stage can belong to arbitrarily many mutex groups
• You can call the mutex groups whatever you want, so that they make sense to you

I'd be happy to contribute a follow-up PR with the mutex feature, if you like the idea

[codecov]

Codecov Report

❌ Patch coverage is 97.90210% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.99%. Comparing base (2431ec6) to head (3a821ca).
⚠️ Report is 175 commits behind head on main.

Files with missing lines	Patch %	Lines
dvc/repo/reproduce.py	95.89%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10922      +/-   ##
==========================================
+ Coverage   90.68%   90.99%   +0.31%     
==========================================
  Files         504      505       +1     
  Lines       39795    41112    +1317     
  Branches     3141     3255     +114     
==========================================
+ Hits        36087    37410    +1323     
- Misses       3042     3064      +22     
+ Partials      666      638      -28     

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[sjawhar]

I think there are some flaky tests :(