GitLab

* [examples/seq2seq] fix t5 examples

This PR:
* fixes T5 examples to include `--source_prefix` - it's **not** optional. If you give it a try you will see that you get 10x worse bleu scores w/o it. w/ `27.6849`, w/ `2.374`
* added a normal translation example w/o the peculiarities of MBart and T5
* reduces the default max samples to 50 so it's much faster to test quickly

summarization seems to be broken for t5 score-wise: https://github.com/huggingface/transformers/issues/10733

@sgugger

* specify explicitly the t5 models requiring the special handling

* one more

* update the t5 summarization example to use cnn_dailymail

* move max*samples into the top level README.md

* better wording

* better wording