fix deprecated ref to `tokenizer.max_len` (#10220)
fix deprecated ref to `tokenizer.max_len` (#10220)

This is to fix deprecated reference to `tokenizer.max_len` with `tokenizer.model_max_length` - similar to [issue 8739](https://github.com/huggingface/transformers/issues/8739) and [PR 8604](https://github.com/huggingface/transformers/pull/8604). 
Example [here](https://colab.research.google.com/gist/poedator/f8776349e5c625ce287fc6fcd312fa1e/tokenizer-max_len-error-in-transformers_glue.ipynb). 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. This simple fix should  do it.
huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers