Polish executor empty program hint message (#22167)

* polish executor hint message, test=develop

* fix punctuation error, test=develop

python/paddle/fluid/executor.py

@@ -792,9 +792,15 @@ class Executor(object):
             program = default_main_program()
         if isinstance(program, Program) and \
                         len(program.global_block().ops) == 0:
-            error_info = "The current program is empty."
             if use_default_main_program:
-                error_info += " Maybe you should pass the Program or the CompiledProgram manually."
+                error_info = "Now you are using default_main_program, "\
+                    "but there are no operators in the program to be executed. "\
+                    "Please ensure you create model correctly or you can pass "\
+                    "the Program or the CompiledProgram manually."
+            else:
+                error_info = "There are no operators in the program to be executed. "\
+                    "If you pass Program manually, please use fluid.program_guard "\
+                    "to ensure the current Program is being used."
             warnings.warn(error_info)
 
         if scope is None: