test: replacing tmpdir with tmp_path in tests_pytorch/conftest.py

[fnhirwa]

What does this PR do?

Part of #16188 (comment)

change tmpdir to tmp_path fixture in tests_pytorch/conftest.py

Fixes #<issue_number>

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📚 Documentation preview 📚: https://pytorch-lightning--19640.org.readthedocs.build/en/19640/

[codecov]

Codecov Report

Merging #19640 (e4eea01) into master (51692c9) will decrease coverage by 31%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19640      +/-   ##
==========================================
- Coverage      84%      53%     -31%     
==========================================
  Files         424      416       -8     
  Lines       34907    34754     -153     
==========================================
- Hits        29351    18436   -10915     
- Misses       5556    16318   +10762