Increase max channles to 64 (#993)

src/include/devcomm.h

@@ -55,7 +55,7 @@ union ncclLLFifoLine {
 };
 
 #define WARP_SIZE warpSize
-#define MAXCHANNELS 32
+#define MAXCHANNELS 64
 #define NCCL_MAX_NTHREADS 256
 #define NCCL_SIMPLE_MAX_NTHREADS NCCL_MAX_NTHREADS
 #define NCCL_LL_MAX_NTHREADS NCCL_MAX_NTHREADS