Polish NCCLHelper

Author: reyoung

paddle/fluid/platform/nccl_helper.h

@@ -61,7 +61,7 @@ struct NCCLContext {
   ncclComm_t comm_;
 
   explicit NCCLContext(int dev_id)
-      : ctx_(new CUDADeviceContext(CUDAPlace(dev_id))) {}
+      : ctx_(new CUDADeviceContext(CUDAPlace(dev_id))), comm_{nullptr} {}
 
   cudaStream_t stream() const { return ctx_->stream(); }
 
@@ -95,6 +95,7 @@ struct NCCLContextMap {
   std::vector<int> order_;
 
   explicit NCCLContextMap(const std::vector<platform::Place> &places) {
+    PADDLE_ENFORCE(!places.empty());
     order_.reserve(places.size());
     for (auto &p : places) {
       int dev_id = boost::get<CUDAPlace>(p).device;
@@ -105,15 +106,17 @@ struct NCCLContextMap {
         order_.size(), contexts_.size(),
         "NCCL Context Map does not support contain two or more same device");
 
-    std::vector<ncclComm_t> comms;
-    comms.resize(order_.size());
+    if (places.size() > 1) {
+      std::vector<ncclComm_t> comms;
+      comms.resize(order_.size());
 
-    PADDLE_ENFORCE(platform::dynload::ncclCommInitAll(
-        &comms[0], static_cast<int>(order_.size()), &order_[0]));
+      PADDLE_ENFORCE(platform::dynload::ncclCommInitAll(
+          &comms[0], static_cast<int>(order_.size()), &order_[0]));
 
-    int i = 0;
-    for (auto &dev_id : order_) {
-      contexts_.at(dev_id).comm_ = comms[i++];
+      int i = 0;
+      for (auto &dev_id : order_) {
+        contexts_.at(dev_id).comm_ = comms[i++];
+      }
     }
   }