Add pre commit config and flake8 config (facebookresearch#2676)

Summary:
# Before submitting
Separating fairinternal/fairseq-py#2212 into separate PR's to be cleaner.

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/main/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: fairinternal/fairseq-py#2676

Reviewed By: alexeib

Differential Revision: D32653505

Pulled By: dianaml0

fbshipit-source-id: fd338289d23b340ef56b0188f9c73b37c367d6ca

.isort.cfg

@@ -0,0 +1,2 @@
+[settings]
+known_third_party = _cffi_backend,agg_results,aml,bitarray,boto3,botocore,dump_hubert_feature,dynamicconv_cuda,editdistance,faiss,fasttext,feature_utils,ffmpeg,g2p_en,h5py,hydra,hypothesis,indicnlp,inflect,iopath,joblib,kaldi_io,kenlm,libfb,librosa,lightconv_cuda,matplotlib,misc,mmpt,mmpt_cli,model,nltk,npy_append_array,numpy,omegaconf,pandas,pathbuilder,preprocessing,progressbar,pythainlp,random_sequence_shuffler,regex,sacrebleu,sacremoses,scipy,sentencepiece,setuptools,six,sklearn,soundfile,sweep,sweep_wmt_en2de_transformer_big_common,tabulate,torch,torchaudio,tqdm,unidecode,utils,videoreader,wav2vec_cluster_faiss,wget,yaml

.pre-commit-config.yaml

@@ -0,0 +1,40 @@
+exclude: 'build|stubs'
+
+default_language_version:
+    python: python3
+
+repos:
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.0.1
+    hooks:
+    -   id: trailing-whitespace
+    -   id: check-ast
+    -   id: check-merge-conflict
+    -   id: no-commit-to-branch
+        args: ['--branch=master']
+    -   id: check-added-large-files
+        args: ['--maxkb=500']
+    -   id: end-of-file-fixer
+
+-   repo: https://github.com/ambv/black
+    rev: 20.8b1
+    hooks:
+    - id: black
+      language_version: python3.8
+
+-   repo: https://gitlab.com/pycqa/flake8
+    rev: 3.9.2
+    hooks:
+    -   id: flake8
+        args: [
+            # only error for syntax errors and undefined names
+            "--select=E9,F63,F7,F82",
+        ]
+
+-   repo: https://github.com/pycqa/isort
+    rev: 5.10.1
+    hooks:
+    -   id: isort
+        exclude: README.md
+        additional_dependencies: [toml]
+        args: ["--profile", "black"]

CONTRIBUTING.md

@@ -26,3 +26,57 @@ clear and has sufficient instructions to be able to reproduce the issue.
 By contributing to Facebook AI Research Sequence-to-Sequence Toolkit (fairseq),
 you agree that your contributions will be licensed under the LICENSE file in
 the root directory of this source tree.
+
+## Pre-commit hooks
+In order to ensure your code lints, there are pre-commit hooks configured in the repository which you can install.
+After installation, they will automatically run each time you commit.
+An abbreviated guide is given below; for more information, refer to [the offical pre-commit documentation](https://pre-commit.com/).
+
+### Installation
+```
+pip install pre-commit
+pre-commit install
+```
+
+### Usage
+Just commit your changes:
+```
+git commit -m "My informative commit message"
+```
+
+If there was a failure, you will get feedback
+```
+[INFO] Initializing environment for https://github.com/PyCQA/flake8.
+[INFO] Installing environment for https://github.com/pre-commit/pre-commit-hooks.
+[INFO] Once installed this environment will be reused.
+[INFO] This may take a few minutes...
+[INFO] Installing environment for https://github.com/PyCQA/flake8.
+[INFO] Once installed this environment will be reused.
+[INFO] This may take a few minutes...
+Trim Trailing Whitespace.................................................Failed
+- hook id: trailing-whitespace
+- exit code: 1
+- files were modified by this hook
+Fixing examples/nllb/modeling/wmt15_benchmark/eval_langs2.sh
+Fix End of Files.........................................................Failed
+- hook id: end-of-file-fixer
+- exit code: 1
+- files were modified by this hook
+Fixing examples/few_shot/scripts/schedule_jobs_few_shot.py
+flake8...................................................................Passed
+```
+
+Certain hooks modify your files to comply.
+To include these modifications, you will need to add them (i.e. `git add ...`) and commit again.
+
+If all is well, you should see something like:
+```
+Trim Trailing Whitespace.................................................Passed
+Fix End of Files.........................................................Passed
+flake8...................................................................Passed
+[gshard-fix-ci 8698644e1] Fix lint, add pre-commit hooks
+ 10 files changed, 148 insertions(+), 110 deletions(-)
+ create mode 100644 .flake8
+ create mode 100644 .pre-commit-config.yaml
+ rename examples/nllb/modeling/wmt15_benchmark/{eval_langs2.py => eval_langs2.sh} (99%)
+ ```

setup.cfg

@@ -0,0 +1,3 @@
+[flake8]
+max-line-length = 127
+extend-ignore = E203, W503