Fix Windows GPU build failure in resize_blinear_op.cc.

I broke it in 67d15573.

Before, I called a function under a `std::is_same<Device, CPUDevice>` condition which cannot be linked if Device is a GPUDevice. I would expect the function not to be generated if Device is a GPUDevice due to dead code elimination, but apparently it still is on Windows.

PiperOrigin-RevId: 327887418
Change-Id: Ib97e1abf1680c75dc072850cc69c761e10ac3e1e

tensorflow/core/kernels/image/resize_bilinear_op.cc

@@ -286,6 +286,25 @@ void resize_image(typename TTypes<T, 4>::ConstTensor images,
   }
 }
 
+template <typename Device>
+struct CastFloatToHalf {
+  void operator()(const Device& d, typename TTypes<float>::ConstFlat input,
+                  typename TTypes<Eigen::half>::Flat output) {
+    output.device(d) = input.template cast<Eigen::half>();
+  }
+};
+
+template <>
+struct CastFloatToHalf<GPUDevice> {
+  void operator()(const GPUDevice& d, typename TTypes<float>::ConstFlat input,
+                  typename TTypes<Eigen::half>::Flat output) {
+    // Use existing cast functor instead of directly casting Eigen tensor, as
+    // otherwise we need to instantiate the cast function in a .cu.cc file
+    functor::CastFunctor<GPUDevice, Eigen::half, float> cast;
+    cast(d, output, input);
+  }
+};
+
 }  // namespace
 
 // Partial specialization of ResizeBilinear functor for a CPUDevice.
@@ -378,19 +397,10 @@ class ResizeBilinearOpGrad : public OpKernel {
       functor::ResizeBilinearGrad<Device, float>()(
           context->eigen_device<Device>(), input_grad, st.height_scale,
           st.width_scale, half_pixel_centers_, output_grad.tensor<float, 4>());
-      if (std::is_same<Device, CPUDevice>::value) {
-        const Device& d = context->template eigen_device<Device>();
-        st.output->template flat<Eigen::half>().device(d) =
-            output_grad.template flat<float>().template cast<Eigen::half>();
-      } else {
-        // Use cast functor instead of directly casting Eigen tensor, as
-        // otherwise we need to instantiate the cast function in a .cu.cc file
-        const Tensor& output_grad_const = output_grad;
-        functor::CastFunctor<Device, Eigen::half, float> cast;
-        const Device& device = context->template eigen_device<Device>();
-        cast(device, st.output->template flat<Eigen::half>(),
-             output_grad_const.template flat<float>());
-      }
+      const Tensor& output_grad_const = output_grad;
+      CastFloatToHalf<Device>{}(context->template eigen_device<Device>(),
+                                output_grad_const.template flat<float>(),
+                                st.output->template flat<Eigen::half>());
     }
   }