Fix downsample channels. (#110)

Reference: https://github.com/huggingface/diffusers/blob/main/src/diffusers/models/unet_2d_blocks.py#L749-L756

This is usually not a problem for Stable Diffusion models. However, some
architectures such as
https://huggingface.co/OFA-Sys/small-stable-diffusion-v0 wouldn't
convert.

python_coreml_stable_diffusion/unet.py

@@ -376,7 +376,7 @@ class CrossAttnDownBlock2D(nn.Module):
         self.resnets = nn.ModuleList(resnets)
 
         if add_downsample:
-            self.downsamplers = nn.ModuleList([Downsample2D(in_channels)])
+            self.downsamplers = nn.ModuleList([Downsample2D(out_channels)])
         else:
             self.downsamplers = None