TorchLean executable model zoo

This module re-exports TorchLean’s small runnable model constructors (MLP/CNN/Transformer/etc.). It is the executable model-zoo counterpart to the pure specs in NN.Spec.Models.*.

The implementations live under NN.GraphSpec.Models.TorchLean.*, because they are architecture constructors. The runtime namespace stays focused on execution machinery: ops, backends, sessions, losses, optimizers, and training loops.

We keep this short NN.TorchLeanModels facade so examples can write TorchLeanModels.mlp without threading a long namespace path everywhere.