'Transforms the input units to (4 * depth) units.
The forget-gate, input-gate, output-gate, and cell update is computed as
f, i, j, o = T(h) + R(x)
where h is hidden units, x is input units, and T, R are transforms of
h, x, respectively.
This method implements R. Note that T is strictly linear, so if LSTM is
going to use bias, this method must include the bias to the transformation.
Subclasses must implement this method. See _Apply() for more details.'

def _TransformInputs(self, _):

    raise NotImplementedError()
