v0.2.4: Patch release

This patch release allows the Stable Diffusion pipelines to be loaded with float16 precision:

pipe = StableDiffusionPipeline.from_pretrained(
           "CompVis/stable-diffusion-v1-4", 
           revision="fp16", 
           torch_dtype=torch.float16, 
           use_auth_token=True
)
pipe = pipe.to("cuda")

The resulting models take up less than 6900 MiB of GPU memory.

• [Loading] allow modules to be loaded in fp16 by @patrickvonplaten in #230