text: Title: Extend Datalab to token classification (entity recognition) datasets
  Body: Allow Datalab to find label issues when `labels` object is token classification annotations (per-word class labels for each sentence/document).
  
  Related things to look at:
  
  Extending Datalab to other ML tasks:
  https://github.com/cleanlab/cleanlab/issues/774
  https://github.com/cleanlab/cleanlab/issues/765
  https://github.com/cleanlab/cleanlab/pull/796
  
  Existing Datalab code for label issues:
  https://github.com/cleanlab/cleanlab/blob/master/cleanlab/datalab/internal/issue_manager/label.py
  
  Adding new issue manager in Datalab:
  https://docs.cleanlab.ai/master/cleanlab/datalab/guide/custom_issue_manager.html
  
  Existing Cleanlab code for token classification label issues:
  https://github.com/cleanlab/cleanlab/tree/master/cleanlab/token_classification