code: def __len__(self) -> int:
      return len(self.channels)
  
imports: import dataclasses
  from abc import abstractmethod
  from typing import Protocol, Tuple, List, Union, Mapping, Callable
  import numpy
  import torch.jit
  from torch import nn, Tensor
  from pytorch_toolbelt.utils import pytorch_toolbelt_deprecated
label_desc: Use image node_id 4 for calling the FeatureMapsSpecification obj's underlying member method code with example usage: obj.__len__() and returns: len