Wait all inputs ready

paddle/fluid/framework/parallel_executor.cc

@@ -375,6 +375,12 @@ struct NCCLAllReduceOpHandle : public OpHandle {
     if (this->inputs_.size() == 1) {
       return;  // No need to all reduce when GPU count = 1;
     } else {
+      // Wait input done
+      for (auto *in : inputs_) {
+        auto &p = static_cast<VarHandle *>(in)->place_;
+        in->generated_op_->Wait(dev_ctx_[p]);
+      }
+
       auto &var_name = static_cast<VarHandle *>(this->inputs_[0])->name_;
       VLOG(3) << "Invoke NCCL AllReduce";
       int dtype = -1;