nn.go held every layer type in the package apart from attention,
recurrence and rope: the Linear and Embedding interfaces with their dense
and quantized types, Conv1d, RMSNorm, LayerNorm and MultiLinear, with one
test file to match. Finding a layer meant scanning the file named after
the package.
Each layer kind gets its own file: linear.go and embedding.go hold the
interface and the dense and quantized types, conv.go, norm.go and
multilinear.go take the rest, and nn_test.go splits the same way. The
Layer and MultiLinearLayer interfaces go; nothing implemented or accepted
them. No code changes otherwise.