Allow us to set the `dtype` (for 'cpu' device)

[jukofyork]

It would be helpful if we could pass the data type instead of defaulting to dtype=torch.bfloat16.

I managed to get this running using the 'cpu' device with OpenMP by hacking the setting to dtype=torch.float32 but with the default dtype=torch.bfloat16 it just sat all night using 1 core and never progressed further (on an older Xeon without native bf16 support, so likely it was trying to upcast into fp32).

---

I also tried preloading the model to use 4bit (with 'cuda' device), which should work for llama models:

        if hf_model is not None:
            hf_cfg = hf_model.config.to_dict()
            qc = hf_cfg.get("quantization_config", {})
            load_in_4bit = qc.get("load_in_4bit", False)
            load_in_8bit = qc.get("load_in_8bit", False)
            quant_method = qc.get("quant_method", "")
            assert not load_in_8bit, "8-bit quantization is not supported"
            assert not (
                load_in_4bit and (version.parse(torch.__version__) < version.parse("2.1.1"))
            ), "Quantization is only supported for torch versions >= 2.1.1"
            assert not (
                load_in_4bit and ("llama" not in model_name.lower())
            ), "Quantization is only supported for Llama models"
            if load_in_4bit:
                assert (
                    qc.get("quant_method", "") == "bitsandbytes"
                ), "Only bitsandbytes quantization is supported"
        else:
            hf_cfg = {}

from: https://github.com/TransformerLensOrg/TransformerLens/blob/main/transformer_lens/HookedTransformer.py

But I got the same shape mismatch exception as mentioned in this thread:

TransformerLensOrg/TransformerLens#569

Might be worth adding the ability to use 4bit if they fix this bug.