Fix fast ParallelExe bug

test=develop

Author: chengduozh

paddle/fluid/framework/details/var_handle.h

@@ -49,6 +49,8 @@ struct VarHandleBase {
 
   void AddOutput(OpHandleBase* out, ir::Node* node) {
     if (pending_ops_.find(out) == pending_ops_.end()) {
+      PADDLE_ENFORCE(out != nullptr, "The output of %s should not be nullptr",
+                     this->Node()->Name());
       pending_ops_.insert(out);
       node_->outputs.push_back(node);
     }

paddle/fluid/framework/parallel_executor.cc

@@ -299,6 +299,12 @@ void ParallelExecutor::FeedAndSplitTensorIntoLocalScopes(
 }
 
 ParallelExecutor::~ParallelExecutor() {
+  const auto dev_ctxs =
+      platform::DeviceContextPool::Instance().GetAllDeviceContexts();
+  for (auto &dev_ctx : dev_ctxs) {
+    dev_ctx->Wait();
+  }
+
   if (member_->own_local_scope_) {
     for (size_t i = 1; i < member_->local_scopes_.size(); ++i) {
       Scope *local_scope = member_->local_scopes_[i];

paddle/fluid/platform/device_context.cc

@@ -35,6 +35,16 @@ platform::DeviceContext* DeviceContextPool::Get(const platform::Place& place) {
   return it->second.get();
 }
 
+const std::vector<const DeviceContext*>
+DeviceContextPool::GetAllDeviceContexts() const {
+  std::vector<const DeviceContext*> all_device_ctx;
+  all_device_ctx.reserve(device_contexts_.size());
+  for (auto& dev_ctx : device_contexts_) {
+    all_device_ctx.emplace_back(dev_ctx.second.get());
+  }
+  return all_device_ctx;
+}
+
 DeviceContextPool::DeviceContextPool(
     const std::vector<platform::Place>& places) {
   PADDLE_ENFORCE_GT(places.size(), 0);

paddle/fluid/platform/device_context.h

@@ -217,6 +217,9 @@ class DeviceContextPool {
   /*! \brief  Return handle of single device context. */
   platform::DeviceContext* Get(const platform::Place& place);
 
+  /*! \brief  Return all the device contexts. */
+  const std::vector<const DeviceContext*> GetAllDeviceContexts() const;
+
   template <typename Place>
   const typename DefaultDeviceContextType<Place>::TYPE* GetByPlace(
       const Place& place) {