Gaussian mixture model (GMM) as an NNModuleSpec

The GMM model spec defines the math for mixture weights, means, and covariances. This file adds a small NNModuleSpec wrapper so the GMM can be composed/exported.

def Spec.GMMModuleSpec {α : Type} [Context α] {nComponents nFeatures : ℕ} (m : GMMSpec α nComponents nFeatures) : ModSpec.NNModuleSpec α (Shape.dim nFeatures Shape.scalar) (Shape.dim nComponents Shape.scalar)

GMM module specification following NNModuleSpec.