add support for safety checker (NSFW filter)

Now you can activate the Hugging Face `diffusers` library safety check
for NSFW and other potentially disturbing imagery.

To turn on the safety check, pass --safety_checker at the command
line. For developers, the flag is `safety_checker=True` passed to
ldm.generate.Generate(). Once the safety checker is turned on, it
cannot be turned off unless you reinitialize a new Generate object.

When the safety checker is active, suspect images will be blurred and
a warning icon is added. There is also a warning message printed in
the CLI, but it can be a little hard to see because of its positioning
in the output stream.

There is a slight but noticeable delay when the safety checker runs.

Note that invisible watermarking is *not* currently implemented. The
watermark code distributed by the CompViz distribution uses a library
that does not seem to be able to retrieve the watermarks it creates,
and it does not appear that Hugging Face `diffusers` or other SD
distributions are doing any watermarking.

environment-mac.yml

@@ -19,6 +19,7 @@ dependencies:
   # ```
   - albumentations==1.2.1
   - coloredlogs==15.0.1
+  - diffusers==0.6.0
   - einops==0.4.1
   - grpcio==1.46.4
   - humanfriendly==10.0