Enable load program state in imperative mode (#24998) (#25441)

* enable load_program_state run in imperative mode; test=develop

* remove useless code; test=develop

Author: phlrain

python/paddle/fluid/io.py

@@ -191,7 +191,8 @@ def _load_program_scope(main=None, startup=None, scope=None):
     with paddle.fluid.scope_guard(scope):
         with paddle.fluid.program_guard(prog, startup_prog):
             with paddle.fluid.unique_name.guard():
-                yield
+                with paddle.fluid.framework._dygraph_guard(None):
+                    yield
 
 
 def _get_valid_program(main_program):

python/paddle/fluid/tests/unittests/test_static_save_load.py

@@ -1153,7 +1153,6 @@ def test_ptb_rnn_cpu_float32(self):
                     # make sure all the paramerter or optimizer var have been set to zero
                     self.assertTrue(np.sum(np.abs(new_t)) == 0)
 
-            #fluid.load(test_program, "./test_1", None )
             program_state = fluid.load_program_state("test_program_1")
             fluid.set_program_state(main_program, program_state)
 
@@ -1164,6 +1163,11 @@ def test_ptb_rnn_cpu_float32(self):
                     base_t = base_map[var.name]
                     self.assertTrue(np.array_equal(new_t, base_t))
 
+        with fluid.dygraph.guard(place):
+            load_state = fluid.load_program_state("test_program_1")
+            for k, v in load_state.items():
+                self.assertTrue(np.array_equal(base_map[k], v))
+
 
 class TestProgramStateOldSaveSingleModel(unittest.TestCase):
     def test_ptb_rnn_cpu_float32(self):