fix batchNorm cpp example

[vsooda]

No description provided.

[piiswrong]

There are a few other places that calls bn. Could you fix those too?

BTW, what's wrong with the original code?

[vsooda]

in cpp-package/include/mxnet-cpp/symbol.h(hpp)

the construct function of Symbol is:

Symbol() {}

inline Symbol::Symbol(SymbolHandle handle) {
  blob_ptr_ = std::make_shared<SymBlob>(handle);
}
inline Symbol::Symbol(const char *name) {
  SymbolHandle handle;
  CHECK_EQ(MXSymbolCreateVariable(name, &(handle)), 0);
  blob_ptr_ = std::make_shared<SymBlob>(handle);
}

so if use the original code,

 Symbol bn = BatchNorm("bn_" + name + suffix, conv, Symbol(), Symbol(), Symbol(), Symbol());

the blob_ptr_ is NULL,

then, in the call stack:

Operator("BatchNorm")->SetInput("gamma", gamma)  
=> SetInput():   input_symbols_.push_back(symbol.GetHandle());
=> symbol.GetHandle() :   blob_ptr_->handle_; //error: blob_ptr_ is NULL 

runtime error occurs.

[vsooda]

I have not found other usage of BatchNorm in the cpp-package

[piiswrong]

ok. Thanks.