code: def norm_l21(A, n_orient, copy=True):
      """L21 norm."""
      if A.size == 0:
          return 0.0
      if copy:
          A = A.copy()
      return np.sum(np.sqrt(groups_norm2(A, n_orient)))
  
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 4 for calling a global function with example usage: norm_l21(A, n_orient, copy) and returns: np, int