[Dy2Stat]Refine ProgramCache.last and Return recent one (#39541)

* Refine ProgramCache.last and Return recent one

* add comment

* fix unittest

python/paddle/fluid/dygraph/dygraph_to_static/program_translator.py

@@ -707,6 +707,8 @@ class ProgramCache(object):
     def __init__(self):
         # {hash_id : (concrete_program, partial_layer)}
         self._caches = collections.OrderedDict()
+        # trace mostly recent used program 
+        self._recent_key = None
 
     def _build_once(self, cache_key):
         concrete_program = ConcreteProgram.from_func_spec(
@@ -722,6 +724,7 @@ class ProgramCache(object):
             raise ValueError('type(item) should be CacheKey, but received %s' %
                              type_name(item))
         item_id = hash(item)
+        self._recent_key = item_id
         if item_id not in self._caches:
             self._caches[item_id] = self._build_once(item)
             # Note: raise warnings if number of traced program is more than `max_tracing_count`
@@ -749,8 +752,8 @@ class ProgramCache(object):
     def last(self):
         assert len(
             self._caches) >= 1, "No valid cached program in ProgramCache."
-        key = next(reversed(self._caches.keys()))
-        return key, self._caches[key]
+        assert self._recent_key is not None
+        return self._recent_key, self._caches[self._recent_key]
 
     def __len__(self):
         return len(self._caches)

python/paddle/fluid/tests/unittests/dygraph_to_static/test_declarative.py

@@ -214,6 +214,7 @@ class TestDifferentInputSpecCacheProgram(unittest.TestCase):
             self.assertTrue(np.allclose(x_data + y_data, out_1.numpy()))
             self.assertTrue(len(foo.program_cache) == 1)
             self.assertTrue(len(foo.program_cache.concrete_programs()) == 1)
+            first_program = foo.program_cache.last()
 
             # [16, 10] + [10] (numpy)
             out_2 = foo(to_variable(x_data), y_data)
@@ -232,6 +233,11 @@ class TestDifferentInputSpecCacheProgram(unittest.TestCase):
             # create a new program
             self.assertTrue(len(foo.program_cache) == 2)
 
+            # test for recent program
+            foo(to_variable(x_data), y_data)
+            recent_program = foo.program_cache.last()
+            self.assertTrue(first_program == recent_program)
+
     def test_get_concrete_program(self):
 
         foo = declarative(foo_func)