Catch Flake8 error codes with multiple letters (pytorch#52750) (pytorch#52801)

Summary:
The Flake8 job has been passing on `master` despite giving warnings for [over a month](https://github.com/pytorch/pytorch/runs/1716124347). This is because it has been using a regex that doesn't recognize error codes starting with multiple letters, such as those used by [flake8-executable](https://pypi.org/project/flake8-executable/). This PR corrects the regex, and also adds another step at the end of the job which asserts that Flake8 actually gave no error output, in case similar regex issues appear in the future.

Tagging the following people to ask what to do to fix these `EXE002` warnings:

- pytorch#50629 authored by jaglinux, approved by rohan-varma
  - `test/distributed/test_c10d.py`
- pytorch#51262 authored by glaringlee, approved by ejguan
  - `torch/utils/data/datapipes/__init__.py`
  - `torch/utils/data/datapipes/iter/loadfilesfromdisk.py`
  - `torch/utils/data/datapipes/iter/listdirfiles.py`
  - `torch/utils/data/datapipes/iter/__init__.py`
  - `torch/utils/data/datapipes/utils/__init__.py`
  - `torch/utils/data/datapipes/utils/common.py`
- pytorch#51398 authored by glaringlee, approved by ejguan
  - `torch/utils/data/datapipes/iter/readfilesfromtar.py`
- pytorch#51599 authored by glaringlee, approved by ejguan
  - `torch/utils/data/datapipes/iter/readfilesfromzip.py`
- pytorch#51704 authored by glaringlee, approved by ejguan
  - `torch/utils/data/datapipes/iter/routeddecoder.py`
  - `torch/utils/data/datapipes/utils/decoder.py`
- pytorch#51709 authored by glaringlee, approved by ejguan
  - `torch/utils/data/datapipes/iter/groupbykey.py`

Specifically, the question is: for each of those files, should we remove the execute permissions, or should we add a shebang? And if the latter, which shebang?

Pull Request resolved: pytorch#52750

Test Plan:
The **Lint / flake8-py3** job in GitHub Actions:

- [this run](https://github.com/pytorch/pytorch/runs/1972039886) failed, showing that the new regex catches these warnings properly
- [this run](https://github.com/pytorch/pytorch/runs/1972393293) succeeded and gave no output in the "Run flake8" step, showing that this PR fixed all Flake8 warnings
- [this run](https://github.com/pytorch/pytorch/pull/52755/checks?check_run_id=1972414849) (in pytorch#52755) failed, showing that the new last step of the job successfully catches Flake8 warnings even without the regex fix

Reviewed By: walterddr, janeyx99

Differential Revision: D26637307

Pulled By: samestep

fbshipit-source-id: 572af6a3bbe57f5e9bd47f19f37c39db90f7b804

Co-authored-by: Sam Estep <[email protected]>

Author: Rong Rong

.github/workflows/lint.yml

@@ -93,9 +93,12 @@ jobs:
           check_name: 'flake8-py3'
           linter_output_path: 'flake8-output.txt'
           commit_sha: ${{ steps.get_pr_tip.outputs.commit_sha }}
-          regex: '^(?<filename>.*?):(?<lineNumber>\d+):(?<columnNumber>\d+): (?<errorCode>\w\d+) (?<errorDesc>.*)'
+          regex: '^(?<filename>.*?):(?<lineNumber>\d+):(?<columnNumber>\d+): (?<errorCode>\w+\d+) (?<errorDesc>.*)'
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      - name: Catch any other warnings
+        run: |
+          [ ! -s flake8-output.txt ]
 
   clang-tidy:
     if: github.event_name == 'pull_request'