plms works, bugs quashed

- The plms sampler now works with custom inpainting model
- Quashed bug that was causing generation on normal models to fail (oops!)
- Can now generate non-square images with custom inpainting model

Credits for advice and assistance during porting:

@any-winter-4079 (http://github.com/any-winter-4079)
@db3000 (Danny Beer http://github.com/db3000)

ldm/models/diffusion/plms.py

@@ -45,7 +45,7 @@ class PLMSSampler(Sampler):
             else:
                 x_in = torch.cat([x] * 2)
                 t_in = torch.cat([t] * 2)
-                c_in = torch.cat([unconditional_conditioning, c])
+                c_in = self.make_cond_in(unconditional_conditioning, c)
                 e_t_uncond, e_t = self.model.apply_model(
                     x_in, t_in, c_in
                 ).chunk(2)