Add possibility to use pythae with lightning library

[iirekm]

The trainer provided by pythae doesn't fit my lightning workflow. When I tried to use it with lightning, it fails, mainly because of the custom, non-standard Dataset pythae uses.

[MisterBourbaki]

Hi there,
Could you provide some more details about what exactly you tried? The BaseDataset used by Pythae is really close to be a very standard Torch Dataset.