Update step size dtypes

[brandonwillard]

In #86, I updated the step size dtypes to int64, but I imagine it would be better to perform that conversion somewhere within AeHMC; however, I don't know the best place for that at the moment.

@rlouf, if you get a minute, tell me where you think an astype could be performed so that users can specify int32 and avoid those Scan casting issues.

Originally posted by @brandonwillard in https://github.com/aesara-devs/aehmc/issues/86#issuecomment-1370220761