Created by: Xreki

FCOp，通过组合MulOp, Row,RowwiseAddOp, SoftmaxOp等实现，在C++实现中通过调用OpRegistry::CreateOp来创建相应的Op。

FCOp的输入为X和W，MulOp的输入为X和Y，在创建MulOp将输入误写为：

    AppendOp(framework::OpRegistry::CreateOp(
             "mul", {{"X", {Input("X")}}, {"W", {Input("W")}}},
             {{"Out", {Output("mul_out")}}}, {}));

单测时报错信息如下：

133: ======================================================================
133: ERROR: test_all (__main__.TestFCOp)
133: ----------------------------------------------------------------------
133: Traceback (most recent call last):
133:   File "/home/liuyiqun01/github/Paddle/python/paddle/v2/framework/tests/op_test_util.py", line 55, in test_all
133:     op = Operator(self.type, **kwargs)
133:   File "/home/liuyiqun01/github/Paddle/build_paddle/build/python/build/lib-python/paddle/v2/framework/op.py", line 161, in __call__
133:     return self.get_op_info(t).method(**kwargs)
133:   File "/home/liuyiqun01/github/Paddle/build_paddle/build/python/build/lib-python/paddle/v2/framework/op.py", line 132, in __impl__
133:     return core.Operator.create(opdesc.SerializeToString())
133: RuntimeError: basic_string::_S_construct null not valid
133: 
133: ----------------------------------------------------------------------

这个报错信息太难以理解了，花了很多时间才发现是MulOp输入的名字写错了，正确的写法应该是：

    AppendOp(framework::OpRegistry::CreateOp(
             "mul", {{"X", {Input("X")}}, {"Y", {Input("W")}}},
             {{"Out", {Output("mul_out")}}}, {}));

重构后的Paddle，需要更友好、直观的错误提示。