Polish fetch error message of parallel executor (#22206)

* polish error message of parallel executor, test=develop

* change PADDLE_ENFORCE, test=develop

paddle/fluid/framework/details/fast_threaded_ssa_graph_executor.cc

@@ -142,8 +142,11 @@ void FastThreadedSSAGraphExecutor::InsertFetchOps(
     PADDLE_ENFORCE_NE(
         fetched_var_it, fetched_vars->end(),
         platform::errors::PreconditionNotMet(
-            "Cannot find fetched variable(%s). Perhaps the main_program "
-            "is not set to ParallelExecutor.",
+            "Cannot find fetched variable(%s) in current computation graph. "
+            "Possible reasons are:\n"
+            "  1. The variable to be fetched is not defined in main program.\n"
+            "  2. The variable to be fetched is not an input or output of any "
+            "operator.",
             var_name));
 
     auto &vars = fetched_var_it->second;

paddle/fluid/framework/details/threaded_ssa_graph_executor.cc

@@ -170,10 +170,15 @@ void ThreadedSSAGraphExecutor::InsertFetchOps(
   for (size_t i = 0; i < fetch_tensors.size(); ++i) {
     auto &var_name = fetch_tensors[i];
     auto fetched_var_it = fetched_vars.find(var_name);
-    PADDLE_ENFORCE(fetched_var_it != fetched_vars.end(),
-                   "Cannot find fetched variable(%s).(Perhaps the main_program "
-                   "is not set to ParallelExecutor)",
-                   var_name);
+    PADDLE_ENFORCE_NE(
+        fetched_var_it, fetched_vars.end(),
+        platform::errors::PreconditionNotMet(
+            "Cannot find fetched variable(%s) in current computation graph. "
+            "Possible reasons are:\n"
+            "  1. The variable to be fetched is not defined in main program.\n"
+            "  2. The variable to be fetched is not an input or output of any "
+            "operator.",
+            var_name));
 
     auto &vars = fetched_var_it->second;