[s2s] clean up + doc (#6184)

Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

examples/seq2seq/finetune.sh

@@ -2,6 +2,7 @@
 export PYTHONPATH="../":"${PYTHONPATH}"
 
 # the proper usage is documented in the README, you need to specify data_dir, output_dir and model_name_or_path
+# run ./finetune.sh --help to see all the possible options
 python finetune.py \
     --learning_rate=3e-5 \
     --fp16 \
@@ -10,8 +11,4 @@ python finetune.py \
     --do_predict \
     --n_val 1000 \
     --val_check_interval 0.1 \
-    --encoder_layerdrop 0.1 \
-    --decoder_layerdrop 0.1 \
-    --dropout 0.1 \
-    --attention_dropout 0.1 \
     $@