text: Title: Ability to add new tokens and keep existing token embeddings frozen?
  Body: ### Feature request
  
  Today, when you add new tokens to the vocabulary (e.g. `<|im_start|>` and `<|im_end|>`), you need to also add `embed_tokens` and `lm_head` to the `modules_to_save` kwarg. This, as far as I can tell, unfreezes all token embeddings.
  
  Would it be possible to unfreeze only the newly added tokens?
  
  ### Motivation
  
  This would allow us to expand the vocabulary, and not fine tune too many extra params.
  
  ### Your contribution
  
  Seeking feedback