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

[ROCm/rccl commit: 6dcae8a459]

projects/rccl/src/include/proxy.h

@@ -61,6 +61,8 @@ struct ncclProxyArgs {
   int idle;
   uint64_t hdp_flushed;
   uint8_t connIndex;
+  uint8_t sendIdx;
+  uint8_t recvIdx;
 
   // Element linking
   pthread_mutex_t mutex;