code: def forward(
      model: Model[NestedT[InItemT], NestedT[OutItemT]],
      Xnest: NestedT[InItemT],
      is_train: bool,
  ) -> Tuple[NestedT[OutItemT], Callable]:
      layer: Model[FlatT[InItemT], FlatT[OutItemT]] = model.layers[0]
      Xflat, lens = _flatten(Xnest)
      Yflat, backprop_layer = layer(Xflat, is_train)
      Ynest = _unflatten(Yflat, lens)
  
      def backprop(dYnest: NestedT[InItemT]) -> NestedT[OutItemT]:
          dYflat, _ = _flatten(dYnest)  # type: ignore[arg-type, var-annotated]
          # type ignore necessary for older versions of Mypy/Pydantic
          dXflat = backprop_layer(dYflat)
          dXnest = _unflatten(dXflat, lens)
          return dXnest
  
      return Ynest, backprop
  
imports: from typing import Any, Callable, List, Optional, Sequence, Tuple, TypeVar, cast
  from ..config import registry
  from ..model import Model
label_desc: Use image node_id 2 for calling a global function with example usage: forward(model, Xnest, is_train) and returns: Ynest,  backprop, dXnest