[FlaxSpeechEncoderDecoder] Fix input shape bug in weights init
The tuple `input_shape` is required in the `init` method of the FlaxSpeechEncoderDecoderModel in order to initialise the model weights - one must specify these input shapes to enable JAX to trace through the model dimensions.
This tuple consists of two entries: the encoder and decoder input lengths. Speech encoders almost always downsample the sequence length dimension. Given an encoder input length, the decoder input length is computed through a convolutional formula. This convolutional formula should take into consideration two convolutional based modules:

1. Feature extractor
2. Adapter module (optional)

Currently, only the first of these two convolutional based modules is accounted for. This PR amends the model script to account for the second of the two, i.e. the adapter module.


_The documentation is not available anymore as the PR was closed or merged._