feat: add constrained decoding to HuggingFaceLM

[zaidsheikh]

Description

Added support for constrained decoding in HuggingFaceLM

References

• Link transformers.GenerationMixin.generate.prefix_allowed_tokens_fn

If provided, this function constraints the beam search to allowed tokens only at each step. If not
provided no constraint is applied. This function takes 2 arguments: the batch ID batch_id and
input_ids. It has to return a list with the allowed tokens for the next generation step conditioned
on the batch ID batch_id and the previously generated tokens inputs_ids. This argument is useful
for constrained generation conditioned on the prefix, as described in Autoregressive Entity
Retrieval.