Fix MLFlowLogger.save_dir Windows file URI handling

[littlebullGit]

Fix MLFlowLogger.save_dir Windows file URI handling

What does this PR do?

Fixes #20972

This PR fixes a bug in MLFlowLogger.save_dir where Windows absolute file URIs were being incorrectly parsed, resulting in malformed local paths that caused FileNotFoundError on Windows systems.

Problem:
When using [MLFlowLogger] with Windows absolute file URIs (e.g., file:///C:/Dev/example/mlruns), the [save_dir] property would return malformed paths like ///C:/Dev/example/mlruns instead of the expected C:/Dev/example/mlruns, causing file system operations to fail.

Solution:

• Replace simple string slicing with proper URI parsing using urllib.parse.urlparse and urllib.request.url2pathname
• Properly handle Windows absolute file URIs (e.g., file:///C:/path)
• Add comprehensive tests for various file URI formats
• Fix malformed paths like ///C:/path becoming C:/path on Windows

Changes:

1. Core Fix: Updated MLFlowLogger.save_dir property to use standard library URI parsing methods
2. Test Coverage: Added comprehensive test [test_mlflow_logger_save_dir_file_uri_handling] covering:
   • Unix-style absolute file URIs
   • Windows-style absolute file URIs
   • Relative file URIs
   • Non-file URIs (should return None)
   • URIs with URL-encoded special characters

---

📚 Documentation preview 📚: https://pytorch-lightning--20988.org.readthedocs.build/en/20988/

[littlebullGit]

@lantiga @williamFalcon , @Borda @ethanwharris @tchaton @justusschock Let me know your comments. Any changes I need to get this PR approved? It has been 2 weeks.

[Borda]

@littlebullGit this requires @williamFalcon only as he is the single code-owner for loggers

[littlebullGit]

The failed check has nothing to do with the code fix. We can rerun the test once @williamFalcon reviewed the code.

[littlebullGit]

just rebased the PR

[littlebullGit]

build error did not seems related to my changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (79ffe50) to head (82b7cbc).
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (79ffe50) and HEAD (82b7cbc). Click for more details.

HEAD has 2815 uploads less than BASE

Flag	BASE (79ffe50)	HEAD (82b7cbc)
cpu	659	30
lightning_fabric	165	0
pytest	330	0
python3.12	197	9
python3.12.7	198	9
lightning	330	15
python3.11	132	6
python3.10	66	3
python	66	3
pytorch2.1	66	6
pytest-full	329	30
pytorch_lightning	164	15
pytorch2.6	33	3
pytorch2.4.1	33	3
pytorch2.3	33	3
pytorch2.2.2	33	3
pytorch2.5.1	33	3
pytorch2.9	33	3
pytorch2.7	33	3
pytorch2.8	32	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20988     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         269      266      -3     
  Lines       23804    23755     -49     
=========================================
- Hits        20626    18732   -1894     
- Misses       3178     5023   +1845