Encoder-Decoder: add informative exception when the decoder is not compatible
# What does this PR do?

Related to https://github.com/huggingface/transformers/issues/23350

Many decoder models are not compatible with our `EncoderDecoder` structure (mostly because using the encoded hidden states appropriately requires extra code in the decoder model architecture itself). This PR adds an informative message in the presence of incompatible decoder models.
_The documentation is not available anymore as the PR was closed or merged._