fix test

the build strategy is finalized after create_passes. So future
change of build strategy has no effects.

test=develop

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

@@ -98,17 +98,18 @@ class TestDistRunnerBase(object):
         strategy.allow_op_delay = False
 
         build_stra = fluid.BuildStrategy()
-        if args.batch_merge_repeat > 1:
-            pass_builder = build_stra._create_passes_from_strategy()
-            mypass = pass_builder.insert_pass(
-                len(pass_builder.all_passes()) - 2, "multi_batch_merge_pass")
-            mypass.set_int("num_repeats", args.batch_merge_repeat)
 
         if args.use_reduce:
             build_stra.reduce_strategy = fluid.BuildStrategy.ReduceStrategy.Reduce
         else:
             build_stra.reduce_strategy = fluid.BuildStrategy.ReduceStrategy.AllReduce
 
+        if args.batch_merge_repeat > 1:
+            pass_builder = build_stra._create_passes_from_strategy()
+            mypass = pass_builder.insert_pass(
+                len(pass_builder.all_passes()) - 2, "multi_batch_merge_pass")
+            mypass.set_int("num_repeats", args.batch_merge_repeat)
+
         exe = fluid.ParallelExecutor(
             args.use_cuda,
             loss_name=avg_cost.name,