Add DNN warning comment (#10368)

export.py

@@ -153,7 +153,7 @@ def export_onnx(model, im, file, opset, dynamic, simplify, prefix=colorstr('ONNX
         f,
         verbose=False,
         opset_version=opset,
-        do_constant_folding=True,
+        do_constant_folding=True,  # WARNING: DNN inference with torch>=1.12 may require do_constant_folding=False
         input_names=['images'],
         output_names=output_names,
         dynamic_axes=dynamic or None)