Add gather, scatter and alltoall collectives

Introducing 3 new APIs:
ncclResult_t  ncclGather(const void* sendbuff, void* recvbuff, size_t sendcount,
    ncclDataType_t datatype, int root, ncclComm_t comm, hipStream_t stream);
ncclResult_t  ncclScatter(const void* sendbuff, void* recvbuff,
    size_t recvcount, ncclDataType_t datatype, int root, ncclComm_t comm,
    hipStream_t stream);
ncclResult_t  ncclAllToAll(const void* sendbuff, void* recvbuff, size_t count,
    ncclDataType_t datatype, ncclComm_t comm, hipStream_t stream);

Only out of place operation is supported.
Preprocessor symbol RCCL_GATHER_SCATTER=1 indicates API availibility.
By default the APIs launche RCCL kernel implementation, which can be disabled by
RCCL_ALLTOALL_KERNEL_DISABLE=1. Then the APIs use wrapper around ncclSend and ncclRecv.


[ROCm/rccl commit: e80e29573c]

projects/rccl/src/include/info.h

@@ -20,7 +20,8 @@ typedef enum {
   ncclPatternTreeDown,
   ncclPatternTreeUpDown,
   ncclPatternCollTreeUp,
-  ncclPatternCollTreeDown
+  ncclPatternCollTreeDown,
+  ncclPatternAll
 } ncclPattern_t;
 
 // Used to pass NCCL call information between functions