Cannot find custom operator when visualizing network.

[wenhe-jia]

When I tried to visualize the mask rcnn network, I used the script below:

import mxnet as mx
sym = mx.sym.load('rcnn1-symbol.json')
mx.viz.plot_network(symbol=sym, node_attrs={"shape": 'oval', "fixedsize": 'false'}).view()

But a error is reported:

/home/jiawenhe/workspace/mxnet/dmlc-core/include/dmlc/logging.h:308: [20:47:29] src/operator/custom/custom.cc:78: Check failed: Registry::Get()->Find(params.op_type) != nullptr Cannot find custom operator MaskOutput

Stack trace returned 10 entries:
[bt] (0) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN4dmlc15LogMessageFatalD1Ev+0x3c) [0x7f4e99987f2c]
[bt] (1) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN5mxnet2op6custom10AttrParserEPN4nnvm9NodeAttrsE+0xaac) [0x7f4e99a5658c]
[bt] (2) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN5mxnet17UpgradeJSON_ParseEN4nnvm5GraphE+0x330) [0x7f4e9b9e1cd0]
[bt] (3) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZNSt17_Function_handlerIFN4nnvm5GraphES1_EPS2_E9_M_invokeERKSt9_Any_dataS1_+0x11f) [0x7f4e9b9e538f]
[bt] (4) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN5mxnet18LoadLegacyJSONPassEN4nnvm5GraphE+0x333) [0x7f4e9b9df9c3]
[bt] (5) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZNSt17_Function_handlerIFN4nnvm5GraphES1_EPS2_E9_M_invokeERKSt9_Any_dataS1_+0x11f) [0x7f4e9b9e538f]
[bt] (6) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN4nnvm11ApplyPassesENS_5GraphERKSt6vectorISsSaISsEE+0x518) [0x7f4e9cf24448]
[bt] (7) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(_ZN4nnvm9ApplyPassENS_5GraphERKSs+0x8e) [0x7f4e9b7c46ae]
[bt] (8) /usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/libmxnet.so(MXSymbolCreateFromFile+0x469) [0x7f4e9b7be799]
[bt] (9) /usr/lib/x86_64-linux-gnu/libffi.so.6(ffi_call_unix64+0x4c) [0x7f4ec7f07adc]

Traceback (most recent call last):
  File "../visual_net.py", line 4, in <module>
    sym = mx.sym.load('/home/jiawenhe/workspace/mx-maskrcnn/rcnn1-symbol.json')
  File "/usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/symbol/symbol.py", line 2494, in load
    check_call(_LIB.MXSymbolCreateFromFile(c_str(fname), ctypes.byref(handle)))
  File "/usr/local/lib/python2.7/dist-packages/mxnet-0.12.0-py2.7.egg/mxnet/base.py", line 146, in check_call
    raise MXNetError(py_str(_LIB.MXGetLastError()))
mxnet.base.MXNetError: [20:47:29] src/operator/custom/custom.cc:78: Check failed: Registry::Get()->Find(params.op_type) != nullptr Cannot find custom operator MaskOutput

This problem also occured with annother custom operator such as RPN, both RPN and MaskOutput are layers implemented by Python. How to fix this?