enable safety_checker for SD 2.1

[anotherjesse]

load the CompVis/stable-diffusion-safety-checker and filter SD2.1 results using it

Update diffusers to more recent version

[anotherjesse]

[anotherjesse]

@bfirsh points out that we should probably raise an exception if there are no generated images - similar to how it worked for SD 1.5

        if len(samples) == 0:
            raise Exception(
                f"NSFW content detected. Try running it again, or try a different prompt."
            )

[anotherjesse]

Here we can see it throws an exception rather than returning an empty list

[anotherjesse]

The exception properly shows NSFW detection

while the default prompt works: