Use IB_MERGE_VFS argument when detecting PCI path

When running in a cloud-hypervisor guest, IB VFs are exposed as a
RCiEP. If the IB VFs are merged, NCCL does not correctly detect
PCI topology.

include/p2p_plugin.h

@@ -123,6 +123,8 @@ int nccl_p2p_ib_speed(int speed);
 
 int64_t ncclParamSharpMaxComms();
 
+int64_t ncclParamIbMergeVfs();
+
 int ncclIbRelaxedOrderingCapable(void);
 
 nccl_p2p_plugin_t nccl_p2p_get_plugin_type();

src/p2p_plugin.c

@@ -385,7 +385,7 @@ ncclResult_t nccl_p2p_ib_pci_path(nccl_ib_dev_t *devs, int num_devs, char* dev_n
     // Merge multi-port NICs into the same PCI device
     p[strlen(p)-1] = '0';
     // Also merge virtual functions (VF) into the same device
-    p[strlen(p)-3] = '0';
+    if (ncclParamIbMergeVfs()) p[strlen(p)-3] = p[strlen(p)-4] = '0';
     // And keep the real port aside (the ibv port is always 1 on recent cards)
     *real_port = 0;
     for (int d=0; d<num_devs; d++) {