Add support architecture HeliumForCausalLM

[jenskastensson]

Helium-1 preview model cannot be directly used with llama.cpp because it uses a new architecture called HeliumForCausalLM.
https://huggingface.co/kyutai/helium-1-preview-2b

[LaurentMazare]

Note that we released Helium 1 which uses the same architecture as llama LlamaForCausalLM so hopefully these should be easier to adapt for running with llama.cpp https://huggingface.co/kyutai/helium-1-2b