Add `examples/run_ner_no_trainer.py`
This PR adds an example of token classification tasks (`"ner", "pos", "chunk"`) to show the functionalities of the new `accelerate` library.

<hr>

**Reviewers:** @sgugger 
Ok, added the missing part in Accelerate. In your code, before gathering the labels and predictions, you should pad them across the processes running if `pad_to_max_length` is False:
```python
if not args.pad_to_max_length:
    predictions = accelerator.pad_across_processes(predictions, dim=1, pad_index=-100)
    labels = accelerator.pad_across_processes(batch["labels"], dim=1, pad_index=-100)
```

This should solve the issue when `pad_to_max_length` is left at `False`!<|||||>Thanks for your comment, @sgugger. I fix the bugs and add documentation to README.<|||||>Thank you for spotting the mistake with unintentionally deleted `--label_all_tokens` from argarser. I added that argument back.<|||||>Just checked on TPU for completeness and it runs perfectly fine there, so we're all good!

Thanks for your contribution!!!