Tokenizer return offsets
# 🚀 Feature request

Request for the feature raised in [Issue #1263](https://github.com/huggingface/transformers/issues/1263).

Previous PRs have attempted to address this but none of them were merged - https://github.com/huggingface/transformers/pull/1274 and https://github.com/huggingface/transformers/pull/2178.

## Motivation

Refer [Issue #1263](https://github.com/huggingface/transformers/issues/1263)

## Your contribution

Please guide me on how to submit a PR.
All fast tokenizers have this feature, just pass along `return_offsets_mapping=True` in your call to the tokenizer. Also note that fast tokenizers are used by default when `AutoTokenizer` is called.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.