Dynamic padding + truncation in classification script
# 🚀 Feature request

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@VictorSanh
@sgugger 

It seems that it is not possible to use dynamic padding along with truncation in classification script, because when `tokenizer` gets `max_length = None` it just skips truncation. 
https://github.com/huggingface/transformers/blob/063d8d27f4e1d089dc76f22e378b86b219167e3b/examples/text-classification/run_glue.py#L290

On the other hand, in language modeling script it works.
https://github.com/huggingface/transformers/blob/063d8d27f4e1d089dc76f22e378b86b219167e3b/examples/language-modeling/run_mlm.py#L311

Yes, the `max_length` should be passed the same way. Would like to open a PR to fix `run_glue.py`?<|||||>Okay, I will do that.