'Transforms the hidden 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 T in the equation. The method must implement a
strictly linear transformation. For example, it may use MatMul or Conv2D,
but must not add bias. This is because when hidden units are zeros, then
the LSTM implementation will skip calling this method, instead of passing
zeros to this function.
Subclasses must implement this method. See _Apply() for more details.'

def _TransformHidden(self, _):

    raise NotImplementedError()
