Mark the lite DeviceDtypeModuleMixin as protected

[carmocca]

What does this PR do?

Follow-up to #14511
This class not meant to be public. Added in a time where we didn't care that much about marking API stability.
No usages out there: https://grep.app/search?q=DeviceDtypeModuleMixin

Does your PR introduce any breaking changes? If yes, please list them.

None

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cc @Borda @akihironitta

[tchaton]

LGTM !