!978 [Data]BUG replace Post() after LaunchWorkers

Merge pull request !978 from xulei/filter_op

mindspore/ccsrc/dataset/engine/datasetops/filter_op.cc

@@ -59,13 +59,13 @@ FilterOp::FilterOp(const std::vector<std::string> &in_col_names, int32_t num_wor
     : ParallelOp(num_workers, op_queue_size), predicate_func_(std::move(predicate_func)), in_columns_(in_col_names) {}
 
 Status FilterOp::operator()() {
-  // Synchronize with TaskManager.
-  TaskManager::FindMe()->Post();
   // The operator class just starts off threads by calling the tree_ function.
   RETURN_UNEXPECTED_IF_NULL(tree_);
   filter_queues_.Init(num_workers_, oc_queue_size_);
   RETURN_IF_NOT_OK(filter_queues_.Register(tree_->AllTasks()));
   RETURN_IF_NOT_OK(tree_->LaunchWorkers(num_workers_, std::bind(&FilterOp::WorkerEntry, this, std::placeholders::_1)));
+  // Synchronize with TaskManager.
+  TaskManager::FindMe()->Post();
   RETURN_IF_NOT_OK(Collector());
   return Status::OK();
 }