*optional*, defaults to 255):
            The index that is ignored by the loss function of the semantic segmentation model.

    Example:

    ```python
    >>> from transformers import MobileNetV2Config, MobileNetV2Model

    >>> # Initializing a "mobilenet_v2_1.0_224" style configuration
    >>> configuration = MobileNetV2Config()

    >>> # Initializing a model from the "mobilenet_v2_1.0_224" style configuration
    >>> model = MobileNetV2Model(configuration)

    >>> # Accessing the model configuration
    >>> configuration = model.config
    ```Z