Add ddp_find_unused_parameters_true alias in Fabric's DDPStrategy

[01AbhiSingh]

What does this PR do?

Fixes #20037

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PR review

Following changes were made in the src/lightning/fabric/strategies/ddp.py :

strategy_registry.register(
    "ddp_find_unused_parameters_true",
    cls,
    description="Alias for `ddp_find_unused_parameters_true` and `start_method='popen'`",
    find_unused_parameters = True,
    start_method = "popen",
)

Tagging @awaelchli for visibility.
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---

📚 Documentation preview 📚: https://pytorch-lightning--20125.org.readthedocs.build/en/20125/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89%. Comparing base (cf24a19) to head (3285d7a).
Report is 29 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #20125   +/-   ##
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  Coverage      89%      89%           
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  Files         267      267           
  Lines       23076    23077    +1     
=======================================
+ Hits        20577    20578    +1     
  Misses       2499     2499           

[awaelchli]

@01AbhiSingh That's great!
Now you need to update the tests, you can see at the bottom of the PR what's failing.

These two tests need to be updated:

FAILED test_cli.py::test_run_get_supported_strategies
FAILED strategies/test_registry.py::test_available_strategies_in_registry

[awaelchli]

Hey @01AbhiSingh are you able to bring this over the finish line? If help is needed, let me know 👍

[01AbhiSingh]

Hey @01AbhiSingh are you able to bring this over the finish line? If help is needed, let me know 👍

Hii @awaelchli. I will update the PR asap, Sorry I got stuck with my end Semester exams. Will get back to you asap.

[01AbhiSingh]

Hii @awaelchli . I tried to understand the remaining two tests that are not passing by looking at the details, but was not able to understand the issue, why is it failing. It would be very helpful if you could help. I was able to clear the tests which you mentioned here below.

@01AbhiSingh That's great! Now you need to update the tests, you can see at the bottom of the PR what's failing.

These two tests need to be updated:

FAILED test_cli.py::test_run_get_supported_strategies
FAILED strategies/test_registry.py::test_available_strategies_in_registry

[awaelchli]

Looks very good, thanks. Well done!