TorchLean Public Trainer Facade

Import entrypoint for the public training API:

let trainer := Trainer.new model
  { task := .regression
    optimizer := optim.adam { lr := 0.03 } }
let y0 ← trainer.eval x
let trained ← trainer.train data { steps := 200, batchSize := 16, logEvery := 25 }
trained.printSummary

Implementation is split into focused modules under NN.API.Public.Facade.Trainer.