[cherry-pick]add cast cuda kernel (#29352) #30263

 add cast cuda kernel

cherry-pick #29352

paddle/fluid/operators/cast_op.cu

@@ -14,6 +14,39 @@ limitations under the License. */
 
 #include "paddle/fluid/operators/cast_op.h"
 #include "paddle/fluid/platform/float16.h"
+#include "paddle/fluid/platform/gpu_launch_config.h"
+
+namespace paddle {
+namespace operators {
+
+template <typename InT, typename OutT>
+__global__ void CastCUDAKernel(const InT* in, const int64_t N, OutT* out) {
+  CUDA_KERNEL_LOOP(index, N) { out[index] = static_cast<OutT>(in[index]); }
+}
+
+template <typename InT>
+struct CastOpFunctor<platform::CUDADeviceContext, InT> {
+  const framework::Tensor* in_;
+  framework::Tensor* out_;
+  const platform::CUDADeviceContext& ctx_;
+  CastOpFunctor(const framework::Tensor* in, framework::Tensor* out,
+                const platform::CUDADeviceContext& ctx)
+      : in_(in), out_(out), ctx_(ctx) {}
+
+  template <typename OutT>
+  void apply() const {
+    auto* in = in_->data<InT>();
+    auto size = in_->numel();
+    auto* out = out_->mutable_data<OutT>(ctx_.GetPlace());
+    platform::GpuLaunchConfig config =
+        platform::GetGpuLaunchConfig1D(ctx_, size);
+    CastCUDAKernel<InT, OutT><<<config.block_per_grid, config.thread_per_block,
+                                0, ctx_.stream()>>>(in, size, out);
+  }
+};
+
+}  // namespace operators
+}  // namespace paddle
 
 namespace ops = paddle::operators;