Introduce multi-rank support per device.

This is a single commit of the source code changes required to
introduce support for multiple ranks per device.
A new interface (ncclCommRankInitMulti) has to be used to make use of
this new feature.

src/include/transport.h

@@ -40,6 +40,7 @@ struct ncclPeerInfo {
   int64_t busId;
   struct ncclComm* comm;
   int cudaCompCap;
+  int virtualId;
 };
 
 #define CONNECT_SIZE 128