rename permutation_invariant_training

[puhuk]

To unify pit to permutation_invariant_training

What does this PR do?

Part of #729, To unify pit to permutation_invariant_training

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[stancld]

Please, bear in mind we need to keep PIT and pit for backward compatibility. Please see #731 :]

---

• deprecation warnings & back compatibility
• add changelog

[codecov]

Codecov Report

Merging #737 (51406af) into master (d36fffc) will decrease coverage by 0%.
The diff coverage is 83%.

@@          Coverage Diff          @@
##           master   #737   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         169    169           
  Lines        6725   6733    +8     
=====================================
+ Hits         6389   6395    +6     
- Misses        336    338    +2     

[stancld]

There're some remaining flake8 issues:

Run flake8 .
./torchmetrics/audio/pit.py:19: [F401] 'torchmetrics.functional.audio.pit.pit' imported but unused
./torchmetrics/audio/pit.py:64: [F811] redefinition of unused 'pit' from line 19
./torchmetrics/audio/pit.py:130: [F811] redefinition of unused 'pit' from line 19

but otherwise, it looks pretty good :]

[Borda]

@stancld fixed now 🐰