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

test/test_CombinedCalls.cpp

@@ -122,6 +122,6 @@ namespace CorrectnessTests
                                 testing::Values(2,3,4,5,6,7,8),
                                 // In-place or not
                                 testing::Values(false),
-                                testing::Values("RCCL_ENABLE_CLIQUE=0", "RCCL_ENABLE_CLIQUE=1")),
+                                testing::Values("RCCL_ENABLE_CLIQUE=0", "RCCL_ENABLE_CLIQUE=1", "RCCL_P2P_NET_DISABLE=0", "RCCL_P2P_NET_DISABLE=1")),
                             CorrectnessTest::PrintToStringParamName());
 } // namespace