Nvtx support (#1076)

* NVTX support

src/include/core.h

@@ -60,6 +60,10 @@ static __inline__ int ncclTypeSize(ncclDataType_t type) {
 #include "alloc.h"
 #include "utils.h"
 #include "param.h"
+#ifdef NVTX_NO_IMPL
 #include "nvtx_stub.h"
+#else
+#include "nvtx.h"
+#endif
 
 #endif // end include guard