optimize program_cache_key, delete cache when use_program_cache is false

Author: jacquesqiao

python/paddle/fluid/executor.py

@@ -260,14 +260,17 @@ def run(self,
             scope = global_scope()
 
         program_cache = None
-        program_cache_key = None
+
+        feed_var_names = feed.keys()
+        fetch_var_names = [var.desc.name() for var in fetch_list]
+        program_cache_key = str(feed_var_names + fetch_var_names)
+
         if use_program_cache:
             # find program cache by cache_key
-            feed_var_names = feed.keys()
-            fetch_var_names = [var.desc.name() for var in fetch_list]
-            program_cache_key = str(feed_var_names + fetch_var_names)
             program_cache = self.program_caches.get(program_cache_key, None)
             # TODO(qiao): Should check program_cache and program are exactly the same.
+        else:
+            self.program_caches.pop(program_cache_key, None)
 
         if program_cache is None:
             program_cache = program.clone()