docstr: Gets called when a stage (either training, validation, test) starts.
code: def on_stage_start(self, stage, epoch=None):
      "Gets called when a stage (either training, validation, test) starts."
      if stage != sb.Stage.TRAIN:
          self.error_metrics = self.hparams.error_stats()
  
file_path: speechbrain/tests/integration/speaker_id/example_xvector_experiment.py
imports: import pathlib
  import speechbrain
  from hyperpyyaml import load_hyperpyyaml
label_desc: Use image node_id 3 for calling the XvectorBrain obj's underlying member method code with example usage: obj.on_stage_start(stage, epoch) without return types