Add support for external network.

Dynamically load external network from libnccl-net.so.
Add init function in networks.
Move PCI scoring to net.cu, only ask transport to provide a path.
Simplify CUDA PCI path detection.
Add dummy external network

ext-net/dummy/Makefile

@@ -0,0 +1,17 @@
+#
+# Copyright (c) 2015-2018, NVIDIA CORPORATION. All rights reserved.
+#
+# See LICENSE.txt for license information
+#
+NCCL_HOME:=../../build/
+CUDA_HOME:=/usr/local/cuda
+INC:= -I$(NCCL_HOME)/include -I$(CUDA_HOME)/include
+PLUGIN_SO:=libnccl-net.so
+
+default: $(PLUGIN_SO)
+
+$(PLUGIN_SO): plugin.c
+	$(CC) $(INC) -fPIC -shared -o $@ -Wl,-soname,$(PLUGIN_SO) $^
+
+clean:
+	rm -f $(PLUGIN_SO)

ext-net/dummy/plugin.c

@@ -0,0 +1,44 @@
+/*************************************************************************
+ * Copyright (c) 2015-2018, NVIDIA CORPORATION. All rights reserved.
+ *
+ * See LICENSE.txt for license information
+ ************************************************************************/
+
+#include <stdio.h>
+#include <nccl.h>
+#include <nccl_net.h>
+
+#define __hidden __attribute__ ((visibility("hidden")))
+
+__hidden ncclResult_t pluginInit() { return ncclSuccess; }
+__hidden ncclResult_t pluginDevices(int* ndev) { *ndev = 0; return ncclSuccess; }
+__hidden ncclResult_t pluginPciPath(int dev, char** path) { return ncclInternalError; }
+__hidden ncclResult_t pluginPtrSupport(int dev, int* supportedTypes) { return ncclInternalError; }
+__hidden ncclResult_t pluginListen(int dev, void* handle, void** listenComm) { return ncclInternalError; }
+__hidden ncclResult_t pluginConnect(int dev, void* handle, void** sendComm) { return ncclInternalError; }
+__hidden ncclResult_t pluginAccept(void* listenComm, void** recvComm) { return ncclInternalError; }
+__hidden ncclResult_t pluginIsend(void* sendComm, void* data, int size, int type, void** request) { return ncclInternalError; }
+__hidden ncclResult_t pluginIrecv(void* recvComm, void* data, int size, int type, void** request) { return ncclInternalError; }
+__hidden ncclResult_t pluginFlush(void* recvComm, void* data, int size) { return ncclInternalError; }
+__hidden ncclResult_t pluginTest(void* request, int* done, int* size) { return ncclInternalError; }
+__hidden ncclResult_t pluginCloseSend(void* sendComm) { return ncclInternalError; }
+__hidden ncclResult_t pluginCloseRecv(void* recvComm) { return ncclInternalError; }
+__hidden ncclResult_t pluginCloseListen(void* listenComm) { return ncclInternalError; }
+
+ncclNet_t NCCL_PLUGIN_SYMBOL = {
+  "Dummy",
+  pluginInit,
+  pluginDevices,
+  pluginPciPath,
+  pluginPtrSupport,
+  pluginListen,
+  pluginConnect,
+  pluginAccept,
+  pluginIsend,
+  pluginIrecv,
+  pluginFlush,
+  pluginTest,
+  pluginCloseSend,
+  pluginCloseRecv,
+  pluginCloseListen
+};