Fix `parent_block.var(name)` error in static mode (#41162)

python/paddle/fluid/layers/control_flow.py

@@ -897,7 +897,9 @@ class StaticRNN(object):
                     if in_var_name not in local_inputs:
                         params.append(in_var_name)
 
-        parameters = [parent_block.var(name) for name in set(params)]
+        parameters = [
+            parent_block._find_var_recursive(name) for name in set(params)
+        ]
 
         step_scope = parent_block.create_var(
             type=core.VarDesc.VarType.STEP_SCOPES)