[pytorch][codegen] simplify python signature creation logic (#47977)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47977
Avoid calling CppSignatureGroup api - python signature shouldn't depend on
cpp signature. Still use cpp.group_arguments() to group TensorOptions.
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: D24976334
Pulled By: ljk53
fbshipit-source-id: 5df5a7bbfd2b8cb460153e5bea4d91e65f716390
