code: def norm_l21_tf(Z, phi, n_orient, w_space=None):
      """L21 norm for TF."""
      if Z.shape[0]:
          l21_norm = np.sqrt(
              phi.norm(Z, ord=2).reshape(-1, n_orient).sum(axis=1)
          )
          if w_space is not None:
              l21_norm *= w_space
          l21_norm = l21_norm.sum()
      else:
          l21_norm = 0.0
      return l21_norm
  
imports: import functools
  from math import sqrt
  import numpy
  from ..time_frequency._stft import istft, stft, stft_norm1, stft_norm2
  from ..utils import _check_option, _get_blas_funcs, _validate_type, logger, sum_squared, verbose, warn
  from .mxne_debiasing import compute_bias
label_desc: Use image node_id 15 for calling a global function with example usage: norm_l21_tf(Z, phi, n_orient, w_space) and returns: l21_norm