Hi wjs20 hope you are having a wonderful day!
Deprecation warnings usually warn of the following scenario.
To mark (a component of a software standard) as obsolete to warn against its use in the future so that it may be phased out.
This link may help resolve the issue, not sure how or where in the code you may need to change your code though.
if the sequences were padded and made the same length would the warning stop?
Is it something that can be configured in fastai?
hope this helps
Cheers mrfabulous1 