Properly handle fp16 branches

[carson-katri]

Fixes an issue where errors would appear trying to load a model that only had a fp16 revision downloaded. This could happen if a user previously used diffusers to download a model outside of the add-on.

A new checkbox is also added to the "Find Models" box to prefer downloading fp16 branches instead of main if possible. This is very useful for users who only run in "Half Precision" and don't need to download the larger weights. If the model doesn't have a fp16 branch, it falls back on whichever branch is default.

[NullSenseStudio]

Mostly good. There's a few issues however.

In hf_snapshot_download()
from diffusers.hub_utils import http_user_agent needs changed to
from diffusers.utils.hub_utils import http_user_agent due to a recent change in diffusers.

[NullSenseStudio]

All good besides a deprecation warning I noticed.