Based on the paper

        Sanchez, Guillaume, et al.
        "Stay on topic with Classifier-Free Guidance."
        arXiv preprint arXiv:2306.17806 (2023).

a draft implementation of classifier free guidance.

This is simply for sharing internally and might very well be
completely wrong. It is debatable if we should expose such
a feature as a flag to the network or make it a separate
classifier instance (or a mixin). In the past we were
very much against special (potentially short-lived) feature
flags and it was much nicer to have this implemented as
an addon/callback. We might need to do something similar
here as well.

- Makes it possible to set gamma parameter
- Setting it to `None` disabled functionality completely

- `label_id` was misleading since it is actually a list of token ids
  related to a label and not a scalar value. Also the general process
  of generating logits it not related to labels at all but rather just
  to tokens

- `kwargs` was named to be similar to transformers `generate`
  convention but is meant to be passed to `generate` and is therefore,
  in the context of `generate_logits` a model input. This should help
  the reader distinguish between expected input (`token_ids`) and
  model input (`model_input`)