Merge pull request #8060 from bosilca/fix/ialltoallw

Prevent some rank from not increasing the non-blocking collective tag if they have no data to exchange.

Author: bosilca

ompi/mca/coll/libnbc/nbc_ialltoallw.c

@@ -67,7 +67,13 @@ static int nbc_alltoallw_init(const void* sendbuf, const int *sendcounts, const
         span = lspan;
       }
     }
+    /**
+     * If this process has no data to send or receive it can bail out early,
+     * but it needs to increase the nonblocking tag to stay in sycn with the
+     * rest of the processses.
+     */
     if (OPAL_UNLIKELY(0 == span)) {
+      ompi_coll_base_nbc_reserve_tags(comm, 1);
       return nbc_get_noop_request(persistent, request);
     }
     tmpbuf = malloc(span);