Chore: Update doc for PyTorch Energy Fitting

[anyangml]

This PR is to perform regression test on PT backend against TF backend, and update doc to provide instructions for using PT backend.

Regression Test on EnergyFitting

Checklist

• Se_a energy training both backends
• Se_r energy training both backends
• dpa1 energy training both backends
• dpa2 energy training pytorch
• backend-convert
  • pb to pth (all 4 models above) se_a,se_r only
  • pth to pb (all 4 models above) se_a,se_r only
• freeze
• test [BUG] SE_R energy fitting tf to pt test #3363

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.99%. Comparing base (665d716) to head (d98ce81).

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #3362   +/-   ##
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  Coverage   75.99%   75.99%           
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  Files         420      420           
  Lines       35291    35291           
  Branches     1618     1618           
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  Hits        26820    26820           
  Misses       7603     7603           
  Partials      868      868           

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