code: def forward(
      model: Model[InT, OutT], Xs: InT, is_train: bool
  ) -> Tuple[OutT, Callable]:
      def backprop(dYr: OutT) -> InT:
          return cast(InT, model.ops.unflatten(dYr.data, dYr.lengths))
  
      lengths = model.ops.asarray1i([len(x) for x in Xs])
      return Ragged(model.ops.flatten(Xs), lengths), backprop
  
imports: from typing import Callable, List, Tuple, TypeVar, cast
  from ..config import registry
  from ..model import Model
  from ..types import ArrayXd, ListXd, Ragged
label_desc: Use image node_id 2 for calling a global function with example usage: forward(model, Xs, is_train) and returns: Ragged,  backprop, cast