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OPT models do not all have a `tokenizer.json` file on the hub at the
moment. Can't merge for now.

closes #99

@njhill, @yk FYI

generated_text was concatenated to the user prompt for legacy reason. We
want to remove this behaviour as we don't think it is useful and even
detrimonial to usability.

We also remove the unused Vec.

- Fix some type hints, in particular base tokenizer class
- Make use of `tensor.new_zero/empty` methods
- Simplify env var string parsing in launcher

Co-authored-by: Nick Hill <nickhill@us.ibm.com>