Do we need Optional in the FlexibleSequential?

[jlperla]

Is the Optional for https://github.com/HighDimensionalEconLab/econ_layers/blob/main/econ_layers/layers.py#L64 and https://github.com/HighDimensionalEconLab/econ_layers/blob/main/econ_layers/layers.py#L66 is necessary for it to work? Typically we would only want the Optional if None also makes sense (e.g. the rescaling_layer needs that, for example, in https://github.com/HighDimensionalEconLab/econ_layers/blob/main/econ_layers/layers.py#L68 Not an important point, but just something to check at some point when you next update the repo.