hf torch_dtype -> dtype

[postylem]

Change torch_dtype -> dtype for huggingface, since they modified that argument recently (see PR huggingface/transformers#39782) resulting in a warning when loading a model

from genlm.backend import load_model_by_name
llm = load_model_by_name("gpt2")
# `torch_dtype` is deprecated! Use `dtype` instead!

[codecov]

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