code: def mask_to_index(mask: Tensor) -> Tensor:
      r"""Converts a mask to an index representation.
  
      Args:
          mask (Tensor): The mask.
  
      Example:
          >>> mask = torch.tensor([False, True, False])
          >>> mask_to_index(mask)
          tensor([1])
      """
      return mask.nonzero(as_tuple=False).view(-1)
  
imports: from typing import Optional
  import torch
  from torch import Tensor
  from torch_geometric.typing import TensorFrame
label_desc: Use image node_id 3 for calling a global function with example usage: mask_to_index(mask) and returns: mask