Added support for XLNet language modelling training in examples
Updated src/transformers/data/data_collator.py with a new XLNet-specific collator that applies masking and generates revelant tensors (input_ids, perm_mask, target_mask, labels) as per https://github.com/zihangdai/xlnet/blob/master/data_utils.py. Also added relevant calls and imports in examples/language-modeling/run_language_modeling.py. Relevant issues #4739 #2008 
CircleCI says:
examples/language-modeling/run_language_modeling.py:29: in <module>
    from transformers import (
E   ImportError: cannot import name 'DataCollatorForXLNetLanguageModeling'

How can I fix this?<|||||>`DataCollatorForXLNetLanguageModeling` needs to be imported into `src/transformers/__init__.py`  to  be imported `from transformers`