Apply lintrunner --all-files -a (#275)

Summary:
Pretty big bang fix for all existing linter issues captured by https://github.com/pytorch/executorch/pull/266.  They are:

* F401 imported but unused
* B028 No explicit stacklevel argument found
* C901 too complex (ignored with `# noqa: C901`)
* C419 Unnecessary list comprehension
* F841 local variable is assigned to but never used
* B023 Function definition does not bind loop variable (only once in `backends/xnnpack/test/test_xnnpack_quantized.py`)
* EXE001 Shebang is present but the file is not executable

### Testing

All tests pass on CI and `lintrunner --all-files -a` passes locally.

Pull Request resolved: https://github.com/pytorch/executorch/pull/275

Test Plan: `arc lint` && `lintrunner --all-files -a` locally

Reviewed By: guangy10

Differential Revision: D49175246

Pulled By: huydhn

fbshipit-source-id: 1d035dd791f9bdafbae14b6d9132e8ca6cf52b9f