Fix SpeechEncoderDecoderModel
# What does this PR do?

The current `SpeechEncoderDecoderModel` doesn't pass the `labels` to the decoder, hence no loss can be calculated. Not sure why it was not included. 

This PR fixes this.

Fixes #13716 


Hey Niels, 

yeah I let this out on purpose because I have to think a bit about the design of the loss function to make it work nicely with the trainer...it's not as straight-forward. Would it be ok to wait with this 1,2 weeks ? :-) <|||||>Hi there! Would the `Seq2SeqTrainer` also need to provide for an `input_values` key [over here](https://github.com/huggingface/transformers/blob/11c69b80452fae4b13c6d8bc22bdc19f3a752199/src/transformers/trainer_seq2seq.py#L168) for it to be compatible when using speech models?<|||||>Hey, 

I will take care of training SpeechEncoderDecoder models next week :-)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Fixed by #14139