[Core] draft_model_runner: Implement prepare_inputs on GPU for advance_step (#6338)

csrc/prepare_inputs/advance_step.cuh

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <torch/all.h>
+
+#include <ATen/cuda/CUDAContext.h>
+#include <c10/cuda/CUDAGuard.h>
+#include <cuda.h>
+#include <cuda_fp16.h>
+#include <cuda_runtime.h>
+#include <iostream>
+
+namespace prepare_inputs {
+
+static constexpr int max_threads = 256;
+static constexpr bool logging = false;
+
+constexpr int div_ceil(int a, int b) { return (a + b - 1) / b; }
+
+}  // namespace prepare_inputs