[Codegen, Cuda] Add overload for fp8x4 e5m2 <-> half4 conversion (#16787)

Author: vinx13

src/target/source/literal/cuda_half_t.h

@@ -410,7 +410,28 @@ struct __align__(8) half4 {
     result.__x =
         (static_cast<__uint32_t>(lo_part.__x) | (static_cast<__uint32_t>(hi_part.__x) << 16));
     return result;
-  })";
+  }
+  __host__ __device__ explicit half4(const __nv_fp8x4_e5m2& fp8x4) {
+    __nv_fp8x2_e5m2 lo_part, hi_part;
+    lo_part.__x = static_cast<__nv_fp8x2_storage_t>(fp8x4.__x & 0xFFFF);
+    hi_part.__x = static_cast<__nv_fp8x2_storage_t>((fp8x4.__x >> 16) & 0xFFFF);
+    __half2 lo_half2 = static_cast<__half2>(lo_part);
+    __half2 hi_half2 = static_cast<__half2>(hi_part);
+    x = reinterpret_cast<__half*>(&lo_half2)[0];
+    y = reinterpret_cast<__half*>(&lo_half2)[1];
+    z = reinterpret_cast<__half*>(&hi_half2)[0];
+    w = reinterpret_cast<__half*>(&hi_half2)[1];
+  }
+  __host__ __device__ explicit operator __nv_fp8x4_e5m2() const {
+    __nv_fp8x4_e5m2 result;
+    __half2 lo_half2 = *reinterpret_cast<const __half2*>(&x);
+    __half2 hi_half2 = *reinterpret_cast<const __half2*>(&z);
+    __nv_fp8x2_e5m2 lo_part(lo_half2), hi_part(hi_half2);
+    result.__x =
+        (static_cast<__uint32_t>(lo_part.__x) | (static_cast<__uint32_t>(hi_part.__x) << 16));
+    return result;
+  }
+  )";
     }
     stream << R"(
 };