[pytorch][codegen] simplify dunder method check in gen_python_functions.py (#47976)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47976
Confirmed byte-for-byte compatible with the old codegen:
```
Run it before and after this PR:
.jenkins/pytorch/codegen-test.sh <baseline_output_dir>
.jenkins/pytorch/codegen-test.sh <test_output_dir>
Then run diff to compare the generated files:
diff -Naur <baseline_output_dir> <test_output_dir>
```
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D24976273
Pulled By: ljk53
fbshipit-source-id: 6f8f20d18db20c3115808bfac0a8b8ad83dcf64c
