title: Enable label alignment for token classification datasets
comments: _The documentation is not available anymore as the PR was closed or merged._ Hmm, not sure why the Windows tests are failing with:
  
  ```
  Did not find path entry C:\tools\miniconda3\bin
  C:\tools\miniconda3\envs\py37\python.exe: No module named pytest
  ```
  
  Edit: running the CI again fixed the problem 🙃  > One last nit and we can merge then
  
  Thanks, done!
body: This PR extends the `Dataset.align_labels_with_mapping()` method to support alignment of label mappings between datasets and models for token classification (e.g. NER).
  
  Example of usage:
  
  ```python
  from datasets import load_dataset
  
  ner_ds = load_dataset("conll2003", split="train")
  # returns [3, 0, 7, 0, 0, 0, 7, 0, 0]
  ner_ds[0]["ner_tags"]
  # hypothetical model mapping with O <--> B-LOC
  label2id = {
      "B-LOC": "0",
      "B-MISC": "7",
      "B-ORG": "3",
      "B-PER": "1",
      "I-LOC": "6",
      "I-MISC": "8",
      "I-ORG": "4",
      "I-PER": "2",
      "O": "5"
    }
  ner_aligned_ds = ner_ds.align_labels_with_mapping(label2id, "ner_tags")
  # returns [3, 5, 7, 5, 5, 5, 7, 5, 5]
  ner_aligned_ds[0]["ner_tags"]
  ```
  
  Context: we need this in AutoTrain to automatically align datasets / models during evaluation. cc @abhishekkrthakur 