tokenize cache for examples/language-modeling
# 🚀 Feature request

I find transformer already has cache for tokenized result in examples/token-classification.
I think language-modeling which with a much larger dataset also need that 

as it takes me about 7 minutes to tokenize a train set (size is 400w) every time I start training. <|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
