Use 2 cards for hallreduce unit test. (#21085)

use 2 cards test=develop

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

@@ -800,7 +800,7 @@ class TestDistBase(unittest.TestCase):
         if self.__use_cuda:
             tr_cmd += " --use_cuda"
             env.update({
-                "CUDA_VISIBLE_DEVICES": "{}".format(trainer_id),
+                "CUDA_VISIBLE_DEVICES": "{}".format(trainer_id % 2),
                 "PADDLE_TRAINERS_NUM": "{}".format(trainer_num),
                 "PADDLE_TRAINER_ID": "{}".format(trainer_id),
                 "PADDLE_TRAINER_ENDPOINTS": self._ps_endpoints,
@@ -813,7 +813,7 @@ class TestDistBase(unittest.TestCase):
             tr_cmd += " --use_dgc"
 
         if self._mp_mode:
-            env = {"FLAGS_selected_gpus": "{}".format(trainer_id)}
+            env = {"FLAGS_selected_gpus": "{}".format(trainer_id % 2)}
 
         if self._nccl_comm_num > 1:
             tr_cmd += " --nccl_comm_num {}".format(self._nccl_comm_num)