Re-enable passing BytesIO as path in .to_onnx()

[GdoongMathew]

What does this PR do?

Fixes #20171

In to_onnx, only convert file_path to string when its a Path instance, and keep io.BytesIO intact and pass it to torch.export.onnx.

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📚 Documentation preview 📚: https://pytorch-lightning--20172.org.readthedocs.build/en/20172/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (a3e60ad) to head (e858928).
Report is 30 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (a3e60ad) and HEAD (e858928). Click for more details.

HEAD has 84 uploads less than BASE

Flag	BASE (a3e60ad)	HEAD (e858928)
cpu	41	21
lightning	30	16
pytest	24	2
python3.9	12	6
python3.11	12	6
gpu	3	2
lightning_fabric	7	0
python3.10	11	6
python3.12	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20172     +/-   ##
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- Coverage      88%      81%     -7%     
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  Files         267      264      -3     
  Lines       23076    23023     -53     
=========================================
- Hits        20280    18613   -1667     
- Misses       2796     4410   +1614     

[awaelchli]

Thanks a lot for the PR! To complete it, we need to add a test. Could we extend the same test as in #19727 by passing in a bytes io? Let me know if you need help with that.

[GdoongMathew]

Hi @awaelchli
Test case was added in that testing function. Thanks.

[awaelchli]

That looks great, thanks for adding the test!