Add num classes comment (facebookresearch#246)

* Add comment explaining num_classes

* More comments

* More comments

models/detr.py

@@ -302,8 +302,18 @@ def forward(self, x):
 
 
 def build(args):
+    # the `num_classes` naming here is somewhat misleading.
+    # it indeed corresponds to `max_obj_id + 1`, where max_obj_id
+    # is the maximum id for a class in your dataset. For example,
+    # COCO has a max_obj_id of 90, so we pass `num_classes` to be 91.
+    # As another example, for a dataset that has a single class with id 1,
+    # you should pass `num_classes` to be 2 (max_obj_id + 1).
+    # For more details on this, check the following discussion
+    # https://github.com/facebookresearch/detr/issues/108#issuecomment-650269223
     num_classes = 20 if args.dataset_file != 'coco' else 91
     if args.dataset_file == "coco_panoptic":
+        # for panoptic, we just add a num_classes that is large enough to hold
+        # max_obj_id + 1, but the exact value doesn't really matter
         num_classes = 250
     device = torch.device(args.device)