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

src/include/param.h

@@ -67,10 +67,10 @@ int64_t ncclParam##name() { \
       errno = 0; \
       int64_t v = strtoll(str, NULL, 0); \
       if (errno) { \
-        INFO(ALL,"Invalid value %s for %s, using default %lu.", str, "NCCL_" env, value); \
+        INFO(NCCL_ALL,"Invalid value %s for %s, using default %lu.", str, "NCCL_" env, value); \
       } else { \
         value = v; \
-        INFO(ALL,"%s set by environment to %lu.", "NCCL_" env, value);  \
+        INFO(NCCL_ALL,"%s set by environment to %lu.", "NCCL_" env, value);  \
       } \
     } \
   } \