add sub-groupnorm in multihead ema, for use in audio modeling

fix laplace activation function thanks to @boweny-cerebras

fix residual within mega layer, thanks to @VHellendoorn

prenorm requires a final layernorm

fix residual for prenorm

expose multi-headed learned EMA, for use outside of repo

offer prenorm architecture

handle bidirectional better

setup enwik8 autoregressive training

improvise on bidirectional for multi-head learned ema