def build_loss(self):
raise NotImplementedError()<EOL>
Implement this function to build the loss function expression.
        Any additional arguments required to build this loss function may be passed in via `__init__`.

        Ideally, the function expression must be compatible with all keras backends and `channels_first` or
        `channels_last` image_data_format(s). `utils.slicer` can be used to define data format agnostic slices.
        (just define it in `channels_first` format, it will automatically shuffle indices for tensorflow
        which uses `channels_last` format).

        ```python
        # theano slice
        conv_layer[:, filter_idx, ...]

        # TF slice
        conv_layer[..., filter_idx]

        # Backend agnostic slice
        conv_layer[utils.slicer[:, filter_idx, ...]]
        ```

        [utils.get_img_shape](vis.utils.utils.md#get_img_shape) is another optional utility that make this easier.

        Returns:
            The loss expression.