fix performance problem caused by Conj (#38939)

paddle/pten/kernels/complex_kernel.h

@@ -14,6 +14,7 @@ limitations under the License. */
 
 #pragma once
 
+#include "paddle/fluid/platform/complex.h"
 #include "paddle/pten/core/dense_tensor.h"
 #include "paddle/pten/infermeta/unary.h"
 #include "paddle/pten/kernels/empty_kernel.h"
@@ -23,7 +24,13 @@ namespace pten {
 template <typename T, typename Context>
 void ConjKernel(const Context& dev_ctx, const DenseTensor& x, DenseTensor* out);
 
-template <typename T, typename Context>
+// If T is complex
+template <typename T,
+          typename Context,
+          std::enable_if_t<
+              std::is_same<T, paddle::platform::complex<float>>::value ||
+                  std::is_same<T, paddle::platform::complex<double>>::value,
+              bool> = true>
 DenseTensor Conj(const Context& dev_ctx, const DenseTensor& x) {
   auto out_meta = UnchangedInferMeta(x.meta());
   auto dense_out = pten::Empty<T, Context>(dev_ctx, std::move(out_meta));
@@ -31,4 +38,15 @@ DenseTensor Conj(const Context& dev_ctx, const DenseTensor& x) {
   return dense_out;
 }
 
+// If T is not complex
+template <typename T,
+          typename Context,
+          std::enable_if_t<
+              !std::is_same<T, paddle::platform::complex<float>>::value &&
+                  !std::is_same<T, paddle::platform::complex<double>>::value,
+              bool> = true>
+DenseTensor Conj(const Context& dev_ctx, const DenseTensor& x) {
+  return x;
+}
+
 }  // namespace pten