class_docstr: Speech task which need generate feat and cmvn
class_docstr_tok: ["Speech","task","which","need","generate","feat","and","cmvn"]
docstr: generate mean and vars of features,
      paths: list or tuple
docstr_tok: ["generate","mean","and","vars","of","features",",","paths",":","list","or","tuple"]
code: def generate_cmvn(self, paths):
      """
      generate mean and vars of features,
          paths: list or tuple
      """
      return NotImplementedError()
  
file_path: delta/delta/data/task/base_task.py
imports: import abc
label_desc: Use image node_id 2 for calling the WavSpeechTask obj's underlying member method code with example usage: obj.generate_cmvn(paths) and returns: NotImplementedError