Put Flake8 requirements into their own file (pytorch#49032)

Summary:
This PR moves the list of Flake8 requirements/versions out of `.github/workflows/lint.yml` and into its own file `requirements-flake8.txt`. After (if) this PR is merged, I'll modify the Flake8 installation instructions on [the "Lint as you type" wiki page](https://github.com/pytorch/pytorch/wiki/Lint-as-you-type) (and its internal counterpart) to just say to install from that new file, rather than linking to the GitHub Actions YAML file and/or giving a command with a set of packages to install that keeps becoming out-of-date.

Pull Request resolved: pytorch#49032

Test Plan:
Either look at CI, or run locally using [act](https://github.com/nektos/act):
```sh
act -P ubuntu-latest=nektos/act-environments-ubuntu:18.04 -j flake8-py3
```

Reviewed By: janeyx99

Differential Revision: D25404037

Pulled By: samestep

fbshipit-source-id: ba4d1e17172a7808435df06cba8298b2b91bb27c

.github/workflows/lint.yml

@@ -75,7 +75,7 @@ jobs:
       - name: Run flake8
         run: |
           set -eux
-          pip install flake8==3.8.2 flake8-bugbear==20.1.4 flake8-comprehensions==3.3.0 flake8-executable==2.0.4 flake8-pyi==20.5.0 mccabe pycodestyle==2.6.0 pyflakes==2.2.0
+          pip install -r requirements-flake8.txt
           flake8 --version
           flake8 | tee ${GITHUB_WORKSPACE}/flake8-output.txt
       - name: Add annotations

CONTRIBUTING.md

@@ -891,7 +891,7 @@ which is in PyTorch's `requirements.txt`.
 ## Pre-commit tidy/linting hook
 
 We use clang-tidy and flake8 (installed with flake8-bugbear,
-flake8-comprehensions, flake8-mypy, and flake8-pyi) to perform additional
+flake8-comprehensions, flake8-pyi, and others) to perform additional
 formatting and semantic checking of code. We provide a pre-commit git hook for
 performing these checks, before a commit is created:
 

requirements-flake8.txt

@@ -0,0 +1,8 @@
+flake8==3.8.2
+flake8-bugbear==20.1.4
+flake8-comprehensions==3.3.0
+flake8-executable==2.0.4
+flake8-pyi==20.5.0
+mccabe
+pycodestyle==2.6.0
+pyflakes==2.2.0