`. For
            positional embeddings use `"absolute"`. For more information on `"relative_key"`, please refer to
            [Self-Attention with Relative Position Representations (Shaw et al.)](https://arxiv.org/abs/1803.02155).
            For more information on `"relative_key_query"`, please refer to *Method 4* in [Improve Transformer Models
            with Better Relative Position Embeddings (Huang et al.)](https://arxiv.org/abs/2009.13658).
        projection_dim (`int`, *optional*, defaults to 0):
            Dimension of the projection for the context and question encoders. If it is set to zero (default), then no
            projection is done.

    Example:

    ```python
    >>> from transformers import DPRConfig, DPRContextEncoder

    >>> # Initializing a DPR facebook/dpr-ctx_encoder-single-nq-base style configuration
    >>> configuration = DPRConfig()

    >>> # Initializing a model (with random weights) from the facebook/dpr-ctx_encoder-single-nq-base style configuration
    >>> model = DPRContextEncoder(configuration)

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