Use fixed segment size for sendrecv (#369)

src/include/devcomm.h

@@ -175,7 +175,7 @@ struct ncclDevComm;
 
 #pragma pack(push)  /* push current alignment to stack */
 #pragma pack(4)     /* set alignment to 4 bytes boundary */
-#define NCCL_MAX_WORK_ELEMENTS 2
+#define NCCL_MAX_WORK_ELEMENTS 1
 #define NCCL_MAX_GROUPS (NCCL_MAX_WORK_ELEMENTS*2)
 
 /* ncclWork is to be a power of two, currently 8x64 bytes, */