class_docstr: Callback that record the mean and std of activations.
docstr: Initialize stats.
docstr_tok: ["Initialize","stats","."]
code: def before_fit(self):
      "Initialize stats."
      super().before_fit()
      self.stats = L()
  
imports: from __future__ import annotations
  from ..basics import *
label_desc: Use image node_id 2 for calling the ActivationStats obj's underlying member method code with example usage: obj.before_fit() without return types