Studio2/SD: Fix sd pipeline up to "Windows not supported"

Amongst other things this address #2063 on Windows, but not necessarily in the correct way

Motivation

Get running the SD pipeline from the SD UI at least to the point where it breaks because I'm on Windows rather than for other reasons.

Remove separate install of iree-runtime and iree-compile in setup_venv.ps1, and rely on the versions installed via the Turbine requirements.txt. Fixes #2063 for me.
Replace any "None" strings with python None when pulling the config in the UI.
Add 'hf_auth_token' param to api StableDiffusion class, defaulting to None, and then pass that in to the various Models where it is required and wasn't already being done before.
Fix clip custom_weight_params being passed to export_clip_model as "external_weight_file" rather than "external_weights"
Don't pass non-existing "custom_vae" parameter to the Turbine Vae Model, instead pass custom_vae as the "hf_model_id" if it is set. (this may be wrong in the custom vae case, but stops the code always breaking).

Looking at the Turbine README, I might have the Fix for #2063 the wrong way round. It's likely that the correct way to do things is to uninstall the version of iree-compiler and iree-runtime that Turbine requires and the do the install from SRT rather than skipping the SRT sourced install.
As usual with changes to setup_venv.ps1 I haven't updated the bash script to match, since I'm not in a position to test it.
As noted, the fix to stop things breaking on the custom_vae parameter not being a thing in the Turbine Model, may not actually make sense if you are using a custom vae.