def forward(
        self, x0: torch.Tensor, x1: torch.Tensor = None, return_features: bool = False
    ):
        """Forward pass through BarlowTwins.

        Extracts features with the backbone and applies the projection
        head to the output space. If both x0 and x1 are not None, both will be
        passed through the backbone and projection. If x1 is None, only x0 will
        be forwarded.
        Barlow Twins only implement a projection head unlike SimSiam.

        Args:
            x0:
                Tensor of shape bsz x channels x W x H.
            x1:
                Tensor of shape bsz x channels x W x H.
            return_features:
                Whether or not to return the intermediate features backbone(x).

        Returns:
            The output projection of x0 and (if x1 is not None)
            the output projection of x1. If return_features is
            True, the output for each x is a tuple (out, f) where f are the
            features before the projection head.

        Examples:
            >>> # single input, single output
            >>> out = model(x)
            >>>
            >>> # single input with return_features=True
            >>> out, f = model(x, return_features=True)
            >>>
            >>> # two inputs, two outputs
            >>> out0, out1 = model(x0, x1)
            >>>
            >>> # two inputs, two outputs with return_features=True
            >>> (out0, f0), (out1, f1) = model(x0, x1, return_features=True)
        """
        # forward pass first input
        f0 = self.backbone(x0).flatten(start_dim=1)
        out0 = self.projection_mlp(f0)

        # append features if requested
        if return_features:
            out0 = (out0, f0)

        if x1 is None:
            return out0

        # forward pass second input
        f1 = self.backbone(x1).flatten(start_dim=1)
        out1 = self.projection_mlp(f1)

        # append features if requested
        if return_features:
            out1 = (out1, f1)

        return out0, out1
Forward pass through BarlowTwins.

        Extracts features with the backbone and applies the projection
        head to the output space. If both x0 and x1 are not None, both will be
        passed through the backbone and projection. If x1 is None, only x0 will
        be forwarded.
        Barlow Twins only implement a projection head unlike SimSiam.

        Args:
            x0:
                Tensor of shape bsz x channels x W x H.
            x1:
                Tensor of shape bsz x channels x W x H.
            return_features:
                Whether or not to return the intermediate features backbone(x).

        Returns:
            The output projection of x0 and (if x1 is not None)
            the output projection of x1. If return_features is
            True, the output for each x is a tuple (out, f) where f are the
            features before the projection head.

        Examples:
            >>> # single input, single output
            >>> out = model(x)
            >>>
            >>> # single input with return_features=True
            >>> out, f = model(x, return_features=True)
            >>>
            >>> # two inputs, two outputs
            >>> out0, out1 = model(x0, x1)
            >>>
            >>> # two inputs, two outputs with return_features=True
            >>> (out0, f0), (out1, f1) = model(x0, x1, return_features=True)
        
lightly/models/barlowtwins.py