Fixed grayscale guide images (#1616)

Fixed grayscale source images

backend/src/nodes/impl/pytorch/pix_transform/auto_split.py

@@ -33,6 +33,12 @@ def split(self, tile_size: Size) -> Size:
         return size, size
 
 
+def _as_3d(img: np.ndarray) -> np.ndarray:
+    if img.ndim == 3:
+        return img
+    return np.expand_dims(img, axis=2)
+
+
 def pix_transform_auto_split(
     source: np.ndarray,
     guide: np.ndarray,
@@ -63,7 +69,7 @@ def upscale(tile: np.ndarray, region: Region):
         try:
             tile_guide = region.scale(scale).read_from(guide)
             pix_op = to_op(PixTransform)(
-                guide_img=np.transpose(tile_guide, (2, 0, 1)),
+                guide_img=np.transpose(_as_3d(tile_guide), (2, 0, 1)),
                 device=device,
                 params=params,
             )