feat : add distributed checkpoint via executor checkpoint_context

- Add checkpoint_context context manager to BaseExecutor with entry/exit barrier
- Add _gather_state_dict hook overridden per executor (template method)
- DDPExecutor skips unwrap on non-rank-0 to avoid redundant state_dict gather
- FSDPExecutor uses rank0_only=True to reduce memory on non-writers
- Remove redundant rank-0 guard from Checkpoint.save and manual barrier from Callback

astrai/serialization/checkpoint.py

@@ -148,9 +148,6 @@ class Checkpoint:
         save_path = Path(save_dir)
         save_path.mkdir(parents=True, exist_ok=True)
 
-        if get_rank() != 0:
-            return
-
         meta = {
             "epoch": self.epoch,
             "consumed_samples": self.consumed_samples,