cudev: add feature to convert FP32(float) from/to FP16(half) on GPU

* add feature of Fp16 on GPU (cudev)
  * add test
  * leave template function as unimplemented to raise error

modules/cudev/include/opencv2/cudev/functional/functional.hpp

@@ -668,6 +668,27 @@ template <typename T, typename D> struct saturate_cast_func : unary_function<T,
     }
 };
 
+// Convert Fp16 dummy
+template <typename T, typename D> struct saturate_cast_fp16_func;
+
+// Convert Fp16 from Fp32
+template <> struct saturate_cast_fp16_func<float, short> : unary_function<float, short>
+{
+    __device__ __forceinline__ short operator ()(float v) const
+    {
+        return cast_fp16<float, short>(v);
+    }
+};
+
+// Convert Fp16 to Fp32
+template <> struct saturate_cast_fp16_func<short, float> : unary_function<short, float>
+{
+    __device__ __forceinline__ float operator ()(short v) const
+    {
+        return cast_fp16<short, float>(v);
+    }
+};
+
 // Threshold Functors
 
 template <typename T> struct ThreshBinaryFunc : unary_function<T, T>

modules/cudev/include/opencv2/cudev/util/saturate_cast.hpp

@@ -270,6 +270,17 @@ template <> __device__ __forceinline__ uint saturate_cast<uint>(double v)
 #endif
 }
 
+template <typename T, typename D> __device__ __forceinline__ D cast_fp16(T v);
+
+template <> __device__ __forceinline__ float cast_fp16<short, float>(short v)
+{
+    return __half2float(v);
+}
+
+template <> __device__ __forceinline__ short cast_fp16<float, short>(float v)
+{
+    return (short)__float2half_rn(v);
+}
 //! @}
 
 }}