Fix PE Unit Test Failure, test=develop (PaddlePaddle#25693)

Based on the comment here https://github.com/PaddlePaddle/Paddle/blob/b5f8784cab94eae785659787fc529870c87b254c/paddle/fluid/framework/details/build_strategy.h#L49

The unit test which compares Reduce and AllReduce must have diff. The PR_CI_Night runs on P40 machine and it has 8GB GPU, which is smaller than the 16GB normal CI machines. So we decrease the batch size in the past to make it runnable: https://github.com/PaddlePaddle/Paddle/pull/24651/files  . Decreasing the batch size makes the difference occurs often. So this PR replace the absolute delta by relative delta.

Before this PR, the unit test failure happens with probability about 1/100. After this PR it doesn't happen.

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

@@ -44,7 +44,7 @@ def _compare_reduce_and_allreduce(self, use_cuda, delta2=1e-5):
         for loss in zip(all_reduce_first_loss, reduce_first_loss):
             self.assertAlmostEquals(loss[0], loss[1], delta=1e-5)
         for loss in zip(all_reduce_last_loss, reduce_last_loss):
-            self.assertAlmostEquals(loss[0], loss[1], delta=delta2)
+            self.assertAlmostEquals(loss[0], loss[1], delta=loss[0] * delta2)
 
         if not use_cuda:
             return
@@ -72,17 +72,17 @@ def _compare_reduce_and_allreduce(self, use_cuda, delta2=1e-5):
         for loss in zip(all_reduce_first_loss, all_reduce_first_loss_seq):
             self.assertAlmostEquals(loss[0], loss[1], delta=1e-5)
         for loss in zip(all_reduce_last_loss, all_reduce_last_loss_seq):
-            self.assertAlmostEquals(loss[0], loss[1], delta=delta2)
+            self.assertAlmostEquals(loss[0], loss[1], delta=loss[0] * delta2)
 
         for loss in zip(reduce_first_loss, reduce_first_loss_seq):
             self.assertAlmostEquals(loss[0], loss[1], delta=1e-5)
         for loss in zip(reduce_last_loss, reduce_last_loss_seq):
-            self.assertAlmostEquals(loss[0], loss[1], delta=delta2)
+            self.assertAlmostEquals(loss[0], loss[1], delta=loss[0] * delta2)
 
         for loss in zip(all_reduce_first_loss_seq, reduce_first_loss_seq):
             self.assertAlmostEquals(loss[0], loss[1], delta=1e-5)
         for loss in zip(all_reduce_last_loss_seq, reduce_last_loss_seq):
-            self.assertAlmostEquals(loss[0], loss[1], delta=delta2)
+            self.assertAlmostEquals(loss[0], loss[1], delta=loss[0] * delta2)
 
 
 class TestResnetWithReduceCPU(TestResnetWithReduceBase):