Add GPUBackend

src/gpu/gpu.cuh

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <cstddef>
+
+void launch_sinf_kernel(const float *d_x, float *d_s, size_t n);
+void launch_cosf_kernel(const float *d_x, float *d_c, size_t n);
+void launch_sincosf_kernel(const float *d_x, float *d_s, float *d_c,
+                           std::size_t n);