Simplify and extend the management of input-conditional losses and updates.

Instead of keeping track of dependencies manually, we rely on the TF graph structure to find dependencies. The resulting implementation is cleaner and more robust.

This does not change any existing behavior. It extends the current behavior by allowing `get_updates_for(inputs)` and `get_losses_for(inputs)` to be called from *any* tensors upstream of the layer, not just the immediate layer's inputs.

PiperOrigin-RevId: 185168680