bug-fix to issue #4042 (#4043)

* allow single-channel-images in draw_bounding_boxes

This is bug-fix for issue #4042.

https://github.com/pytorch/vision/issues/4042

* change made for passing linting requirements

#4042

torchvision/utils.py

@@ -180,6 +180,10 @@ def draw_bounding_boxes(
         raise ValueError("Pass individual images, not batches")
 
     ndarr = image.permute(1, 2, 0).numpy()
+    # allow single-channel-images
+    # shape: (1, H, W) with C = 1
+    if ndarr.shape[-1] == 1:
+        ndarr = np.tile(ndarr, (1, 1, 3))
     img_to_draw = Image.fromarray(ndarr)
 
     img_boxes = boxes.to(torch.int64).tolist()