run_langauge_modeling for T5
Hi
Based on readme on [1], run_langauge_modeling.py does not support T5 model so far, it would be really nice to include this model as well.
There is also this line "data_args.block_size = tokenizer.max_len", max_len does not exist anymore, I searched in pretrainedTokernizer class and did not find an equivalent variable to substitue, do you mind telling me how I can update this line to make this example work? 

thank you.  

[1] https://github.com/huggingface/transformers/blob/master/examples/legacy/run_language_modeling.py
Hi

Seems to me this script is the repetition of this other script: transformers/examples/language-modeling/run_mlm.py 
Do you mind adding T5 also to this script?  thanks <|||||>Actually, we can not simply add T5 to this script, because `run_mlm.py` is for encoder-only models (such as BERT, RoBERTa, DeBERTa, etc.). T5 is an encoder-decoder (seq2seq) model, so this would require a new script. The [seq2seq scripts](https://github.com/huggingface/transformers/tree/master/examples/seq2seq) currently only support fine-tuning, not pre-training.

cc @patil-suraj @sgugger <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.