AttributeError: 'SymbolBlock' object has no attribute '_cached_op_args'

[chansonzhang]

When I run the code in the guide https://nlp.gluon.ai/v0.9.x/examples/sentence_embedding/bert.html#Deployment, it reported AttributeError: 'SymbolBlock' object has no attribute '_cached_op_args' in code line net.export(prefix, epoch=0)

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AttributeError Traceback (most recent call last)
in ()
26 # will remove until mxnet 1.7 release.
27 try:
---> 28 eval_metric = deployment(quantized_net, prefix, dev_dataloader)
29 except NameError:
30 nlp.utils.version.check_version('1.7.0', warning_only=True, library=mx)

1 frames
/usr/local/lib/python3.7/dist-packages/mxnet/gluon/block.py in export(self, path, epoch, remove_amp_cast)
1246 aux_names = set(sym.list_auxiliary_states())
1247 arg_dict = {}
-> 1248 for is_arg, param in self._cached_op_args:
1249 if not is_arg:
1250 name = param.name

AttributeError: 'SymbolBlock' object has no attribute '_cached_op_args'