fix guard place set error (#27573)

python/paddle/fluid/dygraph/base.py

@@ -363,7 +363,7 @@ def guard(place=None):
     with framework.program_guard(train, startup):
         with framework.unique_name.guard():
             with framework._dygraph_guard(tracer):
-                with framework._dygraph_place_guard(place):
+                with framework._dygraph_place_guard(expected_place):
                     yield