Add support for "google/gemma-7b-it"

[kurtome]

HuggingFace now has the Gemma model: https://huggingface.co/google/gemma-7b-it

Expected

    {:ok, nx_model_info} =
      Bumblebee.load_model({:hf, "google/gemma-7b-it", [auth_token: @hf_token]})

    {:ok, nx_tokenizer} =
      Bumblebee.load_tokenizer({:hf, "google/gemma-7b-it", [auth_token: @hf_token]})

Actual

ArgumentError
could not match the class name "GemmaForCausalLM" to any of the supported models,
please specify the :module and :architecture options

[seanmor5]

See: #358