Backend CI on Python 3.11 failed 15 diffusion tests after R30's
accelerate preflight because the CI test environment does not
install accelerate, but the tests mock from_pretrained and never
exercise the CPU-offload path that actually needs it.
Gate the find_spec("accelerate") check on enable_model_cpu_offload
so the dependency is only required for the path that uses it.
transformers preflight stays unconditional (it is always touched
by from_pretrained). Tests with offload=False (the default) pass
without accelerate; production loads with offload=True still get
the fail-fast unload-protection guard the original round-30 fix
added.
97 targeted backend tests pass (test_diffusion_routes,
test_diffusion_backend, test_inference_model_validation,
test_data_recipe_seed, test_training_raw_support,
test_export_log_cursor).
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