deprecated reference `tokenizer.max_len` in glue.py (PR #10220)

[poedator]

There is a deprecated reference to tokenizer.max_len with tokenizer.model_max_length - similar to issue 8739 and PR 8604.
See error example in Colab here. it causes AttributeError: 'BertTokenizer' object has no attribute 'max_len'
The error happens when glue_convert_examples_to_features() is called without max_length parameter specified. In that case line 119 with wrong reference gets called.

I submitted a simple PR #10220. It should be able to fix this issue.

[LysandreJik]

Thanks for opening a PR which fixes the issue! I just merged it.