optimizer.minimize(avg_cost)

报错

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Error Message Summary:
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Error: Operator unknown's GradOpMaker has not been registered.
Please check whether unknown_op has grad_op.
If not, please set stop_gradient to True for its input and output variables using var.stop_gradient=True.
[Hint: grad_op_maker_ should not be null.] at (/paddle/paddle/fluid/framework/op_info.h:77)