message: Update recurrent.py
  trying to edit directly from the browser
diff: @@ -79,10 +79,10 @@ class StackedRNNCells(Layer):
  def __init__(self, cells, **kwargs):
  for cell in cells:
  - if not hasattr(cell, 'call'):
  + if not 'call' in dir(cell):
  raise ValueError('All cells must have a `call` method. '
  'received cells:', cells)
  - if not ('state_size' in dir(cell) or hasattr(cell, 'state_size')):
  + if not 'state_size' in dir(cell):
  raise ValueError('All cells must have a '
  '`state_size` attribute. '
  'received cells:', cells)
  