[BUG]FSDP initialization fails when transformer layer wrap names are stored as a set

[Wangxiaoxiaoa]

Checklist

• The error occurs when using our provided Docker image.
• I can consistently reproduce the bug across multiple trials or random seeds.
• If the error causes experiment abortion, I've verified that this error is the root
  cause, not a secondary error caused by peer workers.

Detailed Information

Describe the bug

FSDP initialization fails with TypeError: 'set' object is not subscriptable.

The failure happens when the transformer layer wrap names are represented as a
set, but the FSDP initialization path assumes the value is indexable.

Expected behavior

FSDP initialization should handle common container types for transformer wrap
names, including set, instead of failing on a type mismatch.

Full logs

Relevant excerpt from the real log:

(AReaL) 20260415-06:21:44.507 EngineBP ERROR: Engine method 'initialize' failed: 'set' object is not subscriptable
Traceback (most recent call last):
  File "areal/infra/rpc/guard/engine_blueprint.py", line 537, in execute_in_engine_thread
    result = method(*args_bcast, **kwargs_bcast)
  File "areal/engine/fsdp_engine.py", line 340, in initialize
    parallelize_model(
  File "areal/engine/fsdp_utils/parallel.py", line 396, in parallelize_model
    apply_fsdp2(model, fsdp_kwargs, wrap_policy)
  File "areal/engine/fsdp_utils/__init__.py", line 80, in apply_fsdp2
    and fsdp_transformer_layer_cls_to_wrap[0] is not None
TypeError: 'set' object is not subscriptable