fix gc transformer ut, test=develop (#23421)

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

@@ -25,6 +25,8 @@ import paddle.dataset.wmt16 as wmt16
 import os
 from feed_data_reader import FeedDataReader
 
+os.environ['CPU_NUM'] = str(4)
+
 
 class ModelHyperParams(object):
     # Dictionary size for source and target language. This model directly uses
@@ -185,10 +187,6 @@ def get_feed_data_reader():
 
 
 class TestTransformer(TestParallelExecutorBase):
-    @classmethod
-    def setUpClass(cls):
-        os.environ['CPU_NUM'] = str(4)
-
     def test_main(self):
         if core.is_compiled_with_cuda():
             self.check_network_convergence(