[Don't Merge][CLI] Add default generator device argument

[gcanlin]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

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[hsliuustc0106]

BLOCKER scan:

• Correctness: PASS
• Reliability/Safety: PASS
• Breaking Changes: PASS
• Test Coverage: PASS (related PRs [Bugfix] Preserve default diffusion sampling params in default stage #2780 and Fix default sampling params to support generator_device in image generations #2778 include tests)
• Documentation: PASS
• Security: PASS

OVERALL: NO BLOCKERS

VERDICT: COMMENT

Small suggestion: Consider adding a unit test for the default_generator_device fallback logic, similar to the generator_device tests in related PRs.