Make size op do no data transformation (#42204)

* fix size op

* update

paddle/fluid/operators/size_op.cc

@@ -23,6 +23,19 @@ namespace operators {
 class SizeOp : public framework::OperatorWithKernel {
  public:
   using framework::OperatorWithKernel::OperatorWithKernel;
+
+ protected:
+  framework::OpKernelType GetExpectedKernelType(
+      const framework::ExecutionContext& ctx) const override {
+    auto dtype = framework::proto::VarType::FP32;  // dtype is not important
+    return framework::OpKernelType(dtype, ctx.GetPlace());
+  }
+
+  framework::OpKernelType GetKernelTypeForVar(
+      const std::string& var_name, const framework::Tensor& tensor,
+      const framework::OpKernelType& expected_kernel_type) const override {
+    return expected_kernel_type;
+  }
 };
 
 class SizeOpMaker : public framework::OpProtoAndCheckerMaker {
@@ -40,6 +53,8 @@ Return the number of elements in the input.
   }
 };
 
+DECLARE_NO_NEED_BUFFER_VARS_INFERER(SizeOpNoNeedBufferVarInferer, "Input");
+
 }  // namespace operators
 }  // namespace paddle
 
@@ -50,4 +65,4 @@ REGISTER_OPERATOR(
     size, ops::SizeOp, ops::SizeOpMaker,
     paddle::framework::EmptyGradOpMaker<paddle::framework::OpDesc>,
     paddle::framework::EmptyGradOpMaker<paddle::imperative::OpBase>,
-    SizeInferShapeFunctor);
+    SizeInferShapeFunctor, ops::SizeOpNoNeedBufferVarInferer);