Select sendrecv path based on collective data size (#391)

* Select sendrecv path based on collective data size

* Add comments on packing and unpacking group field

* Toggling RCCL_P2P_NET_DISABLE in combined calls unit tests

src/include/info.h

@@ -53,6 +53,8 @@ struct ncclInfo {
   int sendChunkSize;
   uint32_t delta;
   int channelId;
+  uint16_t sendIdx;
+  uint16_t recvIdx;
 };
 
 #endif