fix(imperative/dataset): fix VOC dataset

GitOrigin-RevId: a46a4c1173

imperative/python/megengine/data/dataset/vision/voc.py

@@ -184,3 +184,27 @@ class PascalVOC(VisionDataset):
         "train",
         "tvmonitor",
     )
+
+    class_colors = [
+        [0, 0, 0],  # background
+        [0, 0, 128],
+        [0, 128, 0],
+        [0, 128, 128],
+        [128, 0, 0],
+        [128, 0, 128],
+        [128, 128, 0],
+        [128, 128, 128],
+        [0, 0, 64],
+        [0, 0, 192],
+        [0, 128, 64],
+        [0, 128, 192],
+        [128, 0, 64],
+        [128, 0, 192],
+        [128, 128, 64],
+        [128, 128, 192],
+        [0, 64, 0],
+        [0, 64, 128],
+        [0, 192, 0],
+        [0, 192, 128],
+        [128, 64, 0],
+    ]