add annotation

Author: FeixLiu

paddle/fluid/distributed/fleet_executor/carrier.h

@@ -75,6 +75,9 @@ class Carrier final {
 
  bool IsInit() const;
 
+ // NOTE: This mutex will be used in interceptor's RunOps function.
+ // This mutex is used for avoiding forward ops and backward ops run
+ // simultaneously, which will lead to a random hang for some sync ops.
  std::mutex run;
 
  DISABLE_COPY_AND_ASSIGN(Carrier);