docstr: Gets called when a stage starts.
  
  Arguments
  ---------
  stage : Stage
      The stage of the experiment: Stage.TRAIN, Stage.VALID, Stage.TEST
  epoch : int
      The current epoch count.
docstr_tok: ["Gets","called","when","a","stage","starts",".","Arguments","--","--","--","--","-","stage",":","Stage","The","stage","of","the","experiment",":","Stage.TRAIN",",","Stage.VALID",",","Stage.TEST","epoch",":","int","The","current","epoch","count","."]
code: def on_stage_start(self, stage, epoch):
      """Gets called when a stage starts.
  
      Arguments
      ---------
      stage : Stage
          The stage of the experiment: Stage.TRAIN, Stage.VALID, Stage.TEST
      epoch : int
          The current epoch count.
      """
      if stage != sb.Stage.TRAIN:
          if (
              stage == sb.Stage.VALID
              and epoch % self.hparams.evaluation_interval != 0
          ):
              return
  
          self.acc_metric = self.hparams.acc_computer()
          self.bleu_metric = self.hparams.bleu_computer()
          self.last_batch = None
  
          logger.info("Loading pretrained HiFi-GAN ...")
          self.test_vocoder = UnitHIFIGAN.from_hparams(
              source=self.hparams.vocoder_source,
              savedir=self.hparams.vocoder_download_path,
              run_opts={"device": "cpu"},
          )
  
          logger.info("Loading pretrained ASR ...")
          self.test_asr = EncoderDecoderASR.from_hparams(
              source=self.hparams.asr_source,
              savedir=self.hparams.asr_download_path,
              run_opts={"device": "cpu"},
          )
  
imports: import sys
  import torch
  import logging
  import pathlib
  from hyperpyyaml import load_hyperpyyaml
  import speechbrain
  from speechbrain.inference.vocoders import UnitHIFIGAN
  from speechbrain.inference.ASR import EncoderDecoderASR
  import tqdm
  import torchaudio
  import numpy
  from torch.nn.parallel import DistributedDataParallel
label_desc: Use image node_id 7 for calling the S2UT obj's underlying member method code with example usage: obj.on_stage_start(stage, epoch) without return types