The projection layer to apply on the input `encoder_hidden_states`. Set it to `None` if
`encoder_hidden_states` is `None`.
added_emb_type (`str`, *optional*, defaults to `prd`): Additional embeddings to condition the model.
Choose from `prd` or `None`. if choose `prd`, it will prepend a token indicating the (quantized) dot
product between the text embedding and image embedding as proposed in the unclip paper
https://arxiv.org/abs/2204.06125 If it is `None`, no additional embeddings will be prepended.