The modifications that I introduced in a previous commit did break
Bert's internal API. I reverted these changes and added more general
classes to handle the encoder-decoder attention case.
There may be a more elegant way to deal with retro-compatibility (I am
not comfortable with the current state of the code), but I cannot see it
right now.
There is currently no way to specify the quey, key and value separately
in the Attention module. However, the decoder's "encoder-decoder
attention" layers take the decoder's last output as a query, the
encoder's states as key and value. We thus modify the existing code so
query, key and value can be added separately.
This obviously poses some naming conventions; `BertSelfAttention` is not
a self-attention module anymore. The way the residual is forwarded is
now awkard, etc. We will need to do some refacto once the decoder is
fully implemented.
Rémi Louf