[seq2seq] MAX_LEN env var for MT commands (#5837)

examples/seq2seq/README.md

@@ -78,7 +78,7 @@ The following command should work on a 16GB GPU:
     --model_name_or_path facebook/bart-large
 ```
 
-*Note*: The following tips mostly apply to summarization finetuning.
+
 
 ### Translation Finetuning
 
@@ -87,6 +87,7 @@ Then you can finetune mbart_cc25 on english-romanian with the following command.
 **Recommendation:** Read and potentially modify the fairly opinionated defaults in `train_mbart_cc25_enro.sh` script before running it. 
 ```bash
 export ENRO_DIR=${PWD}/wmt_en_ro   # may need to be fixed depending on where you downloaded
+export MAX_LEN=128
 export BS=4
 export GAS=8
 ./train_mbart_cc25_enro.sh --output_dir cc25_v1_frozen/