FSDP2 (fully_shard) integration
- Add apply_fully_shard_data_parallel() with auto/manual mode block detection
- FSDP vs DDP loss/grad parity tests
- Distributed test helpers (testing_utils.py)
- is_fsdp_enabled(), is_fsdp_managed_module() utilities
- Minimal FSDP hooks in from_pretrained
- FSDP-aware flash attention check
