fix benchmark on control flow operators. (#12693)

benchmark/python/control_flow/rnn.py

@@ -32,6 +32,7 @@
 _parser.add_argument('--benchmark', choices=["foreach", "while_loop"], required=True)
 _parser.add_argument('--warmup_rounds', type=int, default=20)
 _parser.add_argument('--test_rounds', type=int, default=100)
+_parser.add_argument('--gpu', type=bool, default=False)
 args = _parser.parse_args()
 
 
@@ -66,8 +67,7 @@ def _func(*states):
             loop_vars=states,
             max_iterations=self.length,
         )
-        assert len(out) == 1
-        return out[0]
+        return out
 
 
 def _zeros(shape, ctx):
@@ -124,7 +124,9 @@ def main():
     cell_types = [gluon.rnn.RNNCell,
                   gluon.rnn.GRUCell,
                   gluon.rnn.LSTMCell]
-    ctxs = [mx.cpu(0)] + [mx.gpu(i) for i in _get_gpus()]
+    ctxs = [mx.cpu(0)]
+    if args.gpu:
+        ctxs = ctxs + [mx.gpu(i) for i in _get_gpus()]
     seq_lens = [100]
     batch_sizes = [1, 32]
     hidden_dims = [512]