fix: broadcast ref/old model state_dict for FSDP

- Add broadcast_state_dict to sync state_dict from rank-0 to all ranks
- Fix create_ref_model returning None on non-rank-0 under FSDP
- Fix sync_old_model only updating old_model on rank-0 under FSDP
- Split skip_no_cuda/skip_no_kernel markers and hoist to top-level conftest
- Add distributed tests for broadcast_state_dict and create_ref_model

astrai/parallel/__init__.py

@@ -7,6 +7,7 @@ from astrai.parallel.executor import (
     FSDPExecutor,
     GradientState,
     NoneExecutor,
+    broadcast_state_dict,
     create_ref_model,
 )
 from astrai.parallel.setup import (
@@ -34,4 +35,5 @@ __all__ = [
     "DDPExecutor",
     "FSDPExecutor",
     "create_ref_model",
+    "broadcast_state_dict",
 ]