annotating the return variables

[Abhishek-TyRnT]

Description

added typecheck for lstm.py file.

Motivation and Context
passes mypy conditions
#4399

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (dba666c) 56.64% compared to head (21ea9f4) 56.64%.

Files	Patch %	Lines
onnx/backend/test/case/node/lstm.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5935      +/-   ##
==========================================
- Coverage   56.64%   56.64%   -0.01%     
==========================================
  Files         506      506              
  Lines       30201    30204       +3     
  Branches     4558     4558              
==========================================
  Hits        17108    17108              
- Misses      12268    12271       +3     
  Partials      825      825              

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[justinchuby]

I don't think this is desired as it is unnecessarily verbose.