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[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 79%. Comparing base (9ae18f7) to head (94db03c). :white_check_mark: All tests successful. No failed tests found.

:exclamation: There is a different number of reports uploaded between BASE (9ae18f7) and HEAD (94db03c). Click for more details.

HEAD has 1156 uploads less than BASE

Flag	BASE (9ae18f7)	HEAD (94db03c)
python3.12.7	80	7
lightning_fabric	74	0
pytest	149	0
cpu	289	28
python3.12	89	9
python3.10	30	3
lightning	140	13
python3.11	60	6
python	30	3
pytorch_lightning	75	15
pytorch2.7	15	3
pytest-full	140	28
pytorch2.1	30	6
pytorch2.2.2	15	3
pytorch2.6	15	3
pytorch2.3	15	3
pytorch2.8	15	3
pytorch2.4.1	10	2
pytorch2.9	15	3
pytorch2.5.1	10	2

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20988     +/-   ##
=========================================
- Coverage      86%      79%     -8%     
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  Files         269      266      -3     
  Lines       23809    23760     -49     
=========================================
- Hits        20574    18661   -1913     
- Misses       3235     5099   +1864