Warn if using a CTC+LM model with no decoder (asr pipeline)
When feeding a model to the automatic-speech-recognition pipeline, if the decoder is forgotten, the model is used as normal ctc without language model applied. This is likely to be accidental, and might go unnoticed. To address this, I propose to add a warning.

cc @LysandreJik 
_The documentation is not available anymore as the PR was closed or merged._<|||||>It would be nice to document the `decoder` parameter, because I did not include it because it is not documented anywhere, but this is a separate issue I guess. Closing this pull request based on review feedback.