Add --no-half-vae to default macOS arguments

Apparently the version of PyTorch macOS users are currently at doesn't always handle half precision VAEs correctly. We will probably want to update the default PyTorch version to 2.0 when it comes out which should fix that, and at this point nightly builds of PyTorch 2.0 are going to be recommended for most Mac users. Unfortunately someone has already reported that their M2 Mac doesn't work with the nightly PyTorch 2.0 build currently, so we can add --no-half-vae for now and give users that can install nightly PyTorch 2.0 builds a webui-user.sh configuration that overrides the default.