class_docstr: Global CMVN (cepstral mean and variance normalization). The global mean
  and variance need to be pre-computed and stored in NumPy format (.npz).
code: def __call__(self, x):
      x = np.subtract(x, self.mean)
      x = np.divide(x, self.std)
      return x
  
imports: import numpy
  from fairseq.data.audio.feature_transforms import AudioFeatureTransform, register_audio_feature_transform
label_desc: Use image node_id 4 for calling the GlobalCMVN obj's underlying member method code with example usage: obj.__call__(x) and returns: x