Fix expanding home directory for Trainer's default_root_dir

[asingh9530]

What does this PR do?

If the user provides Trainer(default_root_dir="~/Trainer"), the path doesn't get expanded. Instead, it creates the folder with name ~ in the current working directory.

Fixes #19059

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📚 Documentation preview 📚: https://pytorch-lightning--19179.org.readthedocs.build/en/19179/

[Borda]

lets add note to change log

[awaelchli]

Looks great!

[codecov]

Codecov Report

Merging #19179 (c60d86f) into master (1284713) will decrease coverage by 34%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19179      +/-   ##
==========================================
- Coverage      83%      49%     -34%     
==========================================
  Files         444      436       -8     
  Lines       36974    36821     -153     
==========================================
- Hits        30616    18080   -12536     
- Misses       6358    18741   +12383     

[asingh9530]

@carmocca looks like test_trainer_compiled_model_test is broken, if possible could you check because it shouldn't fail.