    def forward(self, x: torch.Tensor):
        """
        Args:
            x (torch.Tensor): (batch, max_len, feat_dim)

        Returns:
            (torch.Tensor): normalized feature
        """
        x = x - self.mean
        if self.norm_var:
            x = x * self.istd
        return x
Args:
x (torch.Tensor): (batch, max_len, feat_dim)

Returns:
(torch.Tensor): normalized feature
modules/wenet_extractor/transformer/cmvn.py