fix bug in PT speech-encoder-decoder
# What does this PR do?

Currently in the PT speech-encoder-decoder model, if:

- `encoder_outputs is None` and
- `inputs is not None`

then the encoder is _not_ run in the forward pass, and the `encoder_outputs` remain set to `None`. This throws a `TypeError` when taking the `encoder_hidden_states` - this variable is defined by indexing `encoder_outputs`, which is a `NoneType`.

This PR amends the model to handle this case by running the encoder in a forward pass to yield the `encoder_outputs`. The `encoder_outputs` can then be indexed to give the `encoder_hidden_states`.

_The documentation is not available anymore as the PR was closed or merged._<|||||>Think you need to run `make style` once before being able to merge this PR