code: def mask_type(self) -> str:
      return self._mask_type
  
imports: import math
  from abc import ABC, abstractmethod
  from functools import reduce
  import torch
  import torch.nn.functional
  from packaging.version import parse
  from espnet2.enh.layers.complex_utils import complex_norm, is_complex, new_complex_like
  from espnet2.enh.loss.criterions.abs_loss import AbsEnhLoss
label_desc: Use image node_id 3 for calling the FrequencyDomainL1 obj's underlying member method code with example usage: obj.mask_type() and returns: self