'Unroll the RNN for one time step.
Parameters
inputs : sym.Variable
input symbol, 2D, batch * num_units
states : list of sym.Variable
RNN state from previous step or the output of begin_state().
Returns
output : Symbol
Symbol corresponding to the output from the RNN when unrolling
for a single time step.
states : nested list of Symbol
The new state of this RNN after this unrolling.
The type of this symbol is same as the output of begin_state().
This can be used as input state to the next time step
of this RNN.
See Also
begin_state: This function can provide the states for the first time step.
unroll: This function unrolls an RNN for a given number of (>=1) time steps.'

def __call__(self, inputs, states):

    raise NotImplementedError()
