Fix the file extension of model checkpoints uploaded by NeptuneLogger

[senarvi]

What does this PR do?

NeptuneLogger.after_save_checkpoint() will upload the last and the best model checkpoint using

self.run[...].upload(<filepath>)

instead of

with open(<filepath>) as fp:
    self.run[...] = File.from_stream(fp)

In addition to reducing a couple of lines of code, this will cause the file extension of the checkpoint to appear correctly as .ckpt instead of .bin.

The current code already uploads the best k models this way.

To me this seems like a pretty trivial fix, but I can start a discussion if needed. Maybe some of the previous authors of NeptuneLogger can comment if it was written as it is on purpose? @shnela @Raalsky @williamFalcon @kshitij12345

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📚 Documentation preview 📚: https://pytorch-lightning--20581.org.readthedocs.build/en/20581/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (ea59e40) to head (096db84).
Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (ea59e40) and HEAD (096db84). Click for more details.

HEAD has 2247 uploads less than BASE

Flag	BASE (ea59e40)	HEAD (096db84)
cpu	525	24
lightning_fabric	67	0
pytest	266	0
python3.9	132	6
lightning	394	18
python3.10	66	3
python3.11	132	6
python3.12.7	195	9
gpu	2	0
pytorch2.1	99	9
pytest-full	261	24
pytorch2.2.2	33	3
pytorch_lightning	66	6
pytorch2.3	33	3
pytorch2.4.1	31	3
pytorch2.5.1	65	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20581     +/-   ##
=========================================
- Coverage      88%      79%     -9%     
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  Files         267      264      -3     
  Lines       23380    23322     -58     
=========================================
- Hits        20481    18363   -2118     
- Misses       2899     4959   +2060