[isort] add known 3rd party to setup.cfg (#4053)

* add known 3rd party to setup.cfg

* comment

* Update CONTRIBUTING.md

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

examples/summarization/t5/evaluate_cnn.py

@@ -2,9 +2,9 @@ import argparse
 from pathlib import Path
 
 import torch
+from rouge_score import rouge_scorer, scoring
 from tqdm import tqdm
 
-from rouge_score import rouge_scorer, scoring
 from transformers import T5ForConditionalGeneration, T5Tokenizer