Linear regression as an NNModuleSpec

The spec model file (NN/Spec/Models/LinearRegression.lean) defines the math: forward and backward/VJP pieces.

This file provides the small NNModuleSpec wrapper so linear regression can be composed via SpecChain and recognized by export tooling (PyTorch string renderings, dimension metadata, etc.).

def Spec.LinearRegressionModuleSpec {α : Type} [Context α] {inDim : ℕ} (model : LinearRegressionSpec α inDim) : ModSpec.NNModuleSpec α (Shape.dim inDim Shape.scalar) Shape.scalar

Package a fixed-parameter linear regression as a NNModuleSpec.

PyTorch analogy: nn.Linear(inDim, 1).