text: Title: Training fail when using create_mnbn_model() and Sequential()
  Body: When using the following model and `create_mnbn_model()` in mnist example, i got an error.
  
  model:
  ```
  class BNMLP(chainer.Sequential):
  
      def __init__(self, n_units, n_out):
          super().__init__(
              # the size of the inputs to each layer will be inferred
              L.Linear(784, n_units),  # n_in -> n_units
              L.BatchNormalization(n_units),
              L.Linear(n_units, n_units),  # n_units -> n_units
              L.BatchNormalization(n_units),
              L.Linear(n_units, n_out),  # n_units -> n_out
          )
  ```
  
  How to create mnbn model:
  ```
      model = chainermn.links.create_mnbn_model(
          L.Classifier(BNMLP(args.unit, 10)), comm)
  ```