update ParallelExecutor (#17204)

test=develop

python/paddle/fluid/parallel_executor.py

@@ -96,7 +96,19 @@ class ParallelExecutor(object):
 
         if build_strategy is None:
             build_strategy = BuildStrategy()
+
+        # TODO(paddle-dev): trainer_id and num_trainers should be removed from parameter list.
+        if num_trainers != 1 and build_strategy.num_trainers != num_trainers:
+            sys.stderr.write(
+                'The value of build_strategy.num_trainers[%d] is overwritten '
+                'by the passed num_trainers[%d].\n' %
+                (build_strategy.num_trainers, num_trainers))
             build_strategy.num_trainers = num_trainers
+        if trainer_id != 0 and build_strategy.trainer_id != trainer_id:
+            sys.stderr.write(
+                'The value of build_strategy.trainer_id[%d] is overwritten '
+                'by the passed trainer_id[%d].\n' %
+                (build_strategy.trainer_id, trainer_id))
             build_strategy.trainer_id = trainer_id
 
         self._places = framework.cuda_places(
@@ -104,11 +116,11 @@ class ParallelExecutor(object):
         self._scope = scope if scope is not None else executor.global_scope()
 
         if main_program is not None and main_program._enable_dgc:
-            assert num_trainers > 1, "dgc is not useful when num_trainers <= 1"
+            assert build_strategy.num_trainers > 1, "dgc is not useful when num_trainers <= 1"
             assert build_strategy.reduce_strategy == BuildStrategy.ReduceStrategy.AllReduce, "dgc \
                 only used for allreduce"
 
-            assert num_trainers * len(
+            assert build_strategy.num_trainers * len(
                 self._places) > 1, "dgc is not useful for single card training"
             assert use_cuda, "dgc only used under cuda"