Add condition to dropout and ref to isnan

[titaiwangms]

op.Dropout is only enabled when dropout_p is not 0, and added a reference issue discussion about why op.Where and op.IsNaN are needed when attention mask is boolean value.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.06%. Comparing base (e2fe5e7) to head (a43a1e1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/nn.py	0.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2482      +/-   ##
==========================================
- Coverage   70.09%   70.06%   -0.03%     
==========================================
  Files         211      211              
  Lines       25600    25603       +3     
  Branches     2568     2571       +3     
==========================================
- Hits        17944    17940       -4     
- Misses       6771     6776       +5     
- Partials      885      887       +2     

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