change _compute_metrics not to modify callers version

[misko]

Currently the function _compute_metrics() takes as input a dictionary (out) that contains tensors as values. These values are modified inside of _compute_metrics() , which means the callers version of these tensors are also modified.

out=forward(batch)
_compute_metrics(out)
out # no longer the same values as from forward(batch)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 57.27%. Comparing base (ed82408) to head (c23328a).

Files	Patch %	Lines
ocpmodels/trainers/ocp_trainer.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #645      +/-   ##
==========================================
- Coverage   57.28%   57.27%   -0.01%     
==========================================
  Files         109      109              
  Lines       10315    10316       +1     
==========================================
  Hits         5909     5909              
- Misses       4406     4407       +1     

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