Dtype parameterization

[gabrieletijunaityte]

What does this PR do?

• Added dtype parameter per modality, supported through hydra configs.
• Added dtype parameter for prediction head and normalisation layer, to unify model weight dtypes.

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[vdplasthijs]

Shall we parameterise how Tessera Nans are handled via an arg in base dataset?

[gabrieletijunaityte]

@vdplasthijs @robknapen it's ready for a second round of checking 😅