Automatic layer construction + initialization

[Ebanflo42]

We should have utility functions for constructing dense/convolutional layers (eventually more complex layers like LSTM or multihead attention), which take a context, input node identifiers, and initialization instructions, and return node identifiers for the layer output and the parameters.

A basic example of this is visible in the mnist_xla example.

Intializers should use XLA RNGs.

[Ebanflo42]

relevant

As for construction, the main thing missing is convolutions. this should be an easy thing to finish up. the other thing is incorporating XLA RNGs which is a bit more math-heavy but still doable.