Switch to linear channel mapping for 2 nodes (#1777)

src/include/device.h

@@ -280,7 +280,7 @@ inline __host__ uint8_t ncclP2pChannelBaseForRound(struct ncclComm* comm, int p2
 // ncclP2pChannelToPart and ncclP2pChannelForPart are inverses. The device code
 // uses ncclP2pChannelToPart to determine which part "this" channel is responsible for.
 inline __host__ int ncclP2pChannelForPart(int nP2pChannels, int base, int part, int nParts, int nNodes) {
-  if (nNodes > 1) {
+  if (nNodes > 2) {
     // Only works because nP2pChannels is pow2
     int nChannelsLog2 = countOneBits(nP2pChannels-1);
     int delta = reverseBits(part, nChannelsLog2);
@@ -290,7 +290,7 @@ inline __host__ int ncclP2pChannelForPart(int nP2pChannels, int base, int part,
   }
 }
 inline __device__ int ncclP2pChannelToPart(int nP2pChannels, int base, int channel, int nParts, int nNodes) {
-  if (nNodes > 1) {
+  if (nNodes > 2) {
     // Only works because nP2pChannels is pow2
     int nChannelsLog2 = countOneBits(nP2pChannels-1);
     int delta = (channel-base) & (nP2pChannels-1);