epochs = 500
print_steps = 100
train_loss, train_acc = [], []
val_loss, val_acc = [], []
for i in tnrange(epochs):
    tl, ta = step()
    train_loss += [tl]
    train_acc += [ta]
    if (i+1)%print_steps == 0 or i == 0:
        tl, ta = evaluate(idx_train)
        vl, va = evaluate(idx_val)
        val_loss += [vl]
        val_acc += [va]
        print(f'{i+1:6d}/{epochs}: train_loss={tl:.4f}, train_acc={ta:.4f}'+
              f', val_loss={vl:.4f}, val_acc={va:.4f}')```