encode_prompt's signature defaults device to None. Upstream's own callers always
pass it, so this never fires today, but a cached tuple returned on the CPU would
meet a DiT on the card — and the fallback is one expression, where discovering it
later is a device-mismatch traceback from inside a generation.

Falls back the way _get_t5_prompt_embeds does.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>