Restrict compilation conditions of optimized topk kernel (#41153)

* Restrict compilation conditions of optimized topk kernel

* fix

paddle/fluid/operators/top_k_function_cuda.h

@@ -361,7 +361,7 @@ __global__ void KeMatrixTopK(T* output, int output_stride, int64_t* indices,
 }
 
 /*---------------------------Radix TopK Begin------------------*/
-#if defined(PADDLE_WITH_CUDA)
+#if defined(PADDLE_WITH_CUDA) && CUDA_VERSION >= 9000
 constexpr int RADIX_BITS = 2;  // digits are base-(2 ^ RADIX_BITS)
 constexpr int RADIX_SIZE = 4;  // 2 ^ RADIX_BITS
 constexpr int RADIX_MASK = (RADIX_SIZE - 1);
@@ -479,15 +479,25 @@ struct RadixTypeConfig<platform::float16> {
   typedef uint32_t RadixType;
 
   static inline __device__ RadixType Convert(platform::float16 v) {
+#if CUDA_ARCH_FP16_SUPPORTED(__CUDA_ARCH__)
     half v_h = v.to_half();
     RadixType x = __half_as_ushort(v_h);
     RadixType mask = (x & 0x00008000) ? 0x0000ffff : 0x00008000;
     return (v_h == v_h) ? (x ^ mask) : 0xffff;
+#else
+    assert(false);
+    return 0u;
+#endif
   }
 
   static inline __device__ platform::float16 Deconvert(RadixType v) {
+#if CUDA_ARCH_FP16_SUPPORTED(__CUDA_ARCH__)
     RadixType mask = (v & 0x00008000) ? 0x00008000 : 0x0000ffff;
     return static_cast<platform::float16>(__ushort_as_half(v ^ mask));
+#else
+    assert(false);
+    return static_cast<platform::float16>(0);
+#endif
   }
 };