Update the unneeded op to work but warn user

include/ck/tensor_operation/gpu/element/unary_element_wise_operation.hpp

@@ -115,7 +115,10 @@ struct ConvertBF16RTN
     template <>
     __host__ __device__ void operator()<half_t, half_t>(half_t& y, const half_t& x) const
     {
-        y = type_convert<half_t>(x);
+        y = x;
+
+        // fill user screen, but exit gracefully
+        printf("This datatype is not supported, please use ConvertBF16RTN for bf16 data only.\n");
     }
 };