fix perplexity logging

[sichu2023]

Replace callback method in logging perplexity with torchmetrics.

Loss curve is shifted by a constant when TP=2 but gradient is not affected and the perplexity value match with the loss curve without TP. This is a previously unidentified problem and will be resolved by another PR.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.57%. Comparing base (0c990a7) to head (0bd0330).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...-packages/bionemo-llm/src/bionemo/llm/lightning.py	88.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #622      +/-   ##
==========================================
- Coverage   86.62%   86.57%   -0.05%     
==========================================
  Files         116      116              
  Lines        6961     6929      -32     
==========================================
- Hits         6030     5999      -31     
+ Misses        931      930       -1     

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