[s2s] pass max_length to config through command line
Problem:
In summarization, ideal beam search params vary between finetuning datasets. If you are finetuning pegasus-large on xsum, you want config.max_length=56, if you are finetuning pegasus-large on cnn-dailymail you want config.max_length=128. 

### Solutions
- the command line arg should be called `max_generate_length`

- This could also be addressed through adding `task_specific_params` for every dataset. Then you could pass `--task summarize_xsum` to finetune.py and things would work. Kinda lame though.