Follow comments, correct implement `DDim::size()`

* Also fix unit test

paddle/framework/ddim.cc

@@ -278,7 +278,7 @@ std::ostream& operator<<(std::ostream& os, const DDim& ddim) {
   return os;
 }
 
-ssize_t DDim::size() const { return product(*this); }
+ssize_t DDim::size() const { return arity(*this); }
 
 DDim::DDim(std::initializer_list<int> init_list) {
   *this = make_ddim(init_list);

paddle/operators/softmax_op.cu

 #include <paddle/framework/op_registry.h>
-#include <paddle/operators/sigmoid_op.h>
+#include <paddle/operators/softmax_op.h>
 
 REGISTER_OP_GPU_KERNEL(
     softmax, paddle::operators::SoftmaxKernel<paddle::platform::GPUPlace>);