[Fix] Fix num_classes bug in pytorch2onnx.py (#458)

* Fix num_classes bug in pytorch2onnx.py

* Fix `num_classes=-1`.

tools/deployment/pytorch2onnx.py

@@ -59,7 +59,15 @@ def pytorch2onnx(model,
     """
     model.cpu().eval()
 
-    num_classes = model.head.num_classes
+    if hasattr(model.head, 'num_classes'):
+        num_classes = model.head.num_classes
+    # Some backbones use `num_classes=-1` to disable top classifier.
+    elif getattr(model.backbone, 'num_classes', -1) > 0:
+        num_classes = model.backbone.num_classes
+    else:
+        raise AttributeError('Cannot find "num_classes" in both head and '
+                             'backbone, please check the config file.')
+
     mm_inputs = _demo_mm_inputs(input_shape, num_classes)
 
     imgs = mm_inputs.pop('imgs')