[Bug Report] read_sae_from_disk should default to the SAE dtype unless it's overridden

[hijohnnylin]

Describe the bug
sae_lens_loader in pretrained_sae_loaders.py currently does not pass in dtype into read_sae_from_disk, so it always initializes weights using float32 first.

The dtype is correct at the very end, since sae is created/initialized with the correct dtype from cfg_dict. However, ideally we would be using the correct dtype the whole time.

The fix seems to be that read_sae_from_disk should 1) make dtype optional and 2) use the dtype specified by the cfg_dict by default, if dtype is not specified. This has the benefit that other methods calling read_sae_from_disk would have the correct behavior by default, unless they choose to override.

I'll make a PR for this soon.

Checklist

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