Use registered RLix bucket size argument

[TianyeGGBond]

Context

--miles-model-update-bucket-size-mb is registered unconditionally by add_rlix_arguments in miles/utils/arguments.py, with default=512. Since add_rlix_arguments is always applied during argument construction, runtime users can read args.miles_model_update_bucket_size_mb directly.

Two call sites still had defensive fallbacks that repeated the same 512 default:

• MegatronTrainRayActor._ensure_cpu_bucket_cache
• assert_rlix_topology in miles/utils/rlix_validation.py

Those fallbacks are unnecessary and make the default value live in multiple places.

What changed

• Replaced getattr(self.args, miles_model_update_bucket_size_mb, 512) with direct self.args.miles_model_update_bucket_size_mb access in actor.py.
• Replaced getattr(args, miles_model_update_bucket_size_mb, 512) or 512 with direct args.miles_model_update_bucket_size_mb access in rlix_validation.py.
• Left the actual default centralized in arguments.py.

Why

This keeps the bucket-size default in one place and avoids drift if the default changes later. It also matches the argument registration contract: the field is expected to exist after parser setup.

Validation

• python -m py_compile miles/backends/megatron_utils/actor.py miles/utils/rlix_validation.py
• git diff --check
• git grep -n miles_model_update_bucket_size_mb\|miles-model-update-bucket-size-mb -- miles to confirm the remaining default definition is centralized in arguments.py