Datasets & collators for NER
# 🚀 Feature request

Is there a plan to add a dataset subclass and loader/collator for token classification tasks in the CONLL format? Or has that been deliberately avoided for some reason. 

## Motivation

We have TextDatasets and DataCollatorForLanguageModeling. Can we have something similar for token classification tasks with some CONLL format input text file? That way we can just point to a file & tokenizer and generate a dataset like the language modeling tasks.

## Your contribution

I have a working version of this that I use in my projects. I can package and share it if it would be useful to add directly to transformers library.
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.
