[minor] ThreadLocal to ThreadLocalCheckpointState dataclass (#1007)

* ThreadLocal to ThreadLocalCheckpointState dataclass

* remove notes

fairscale/nn/checkpoint/checkpoint_activations.py

@@ -4,6 +4,7 @@
 # LICENSE file in the root directory of this source tree.
 
 from contextlib import contextmanager
+from dataclasses import dataclass
 import functools
 import threading
 from typing import Any, Dict, Generator, Optional, Tuple
@@ -21,14 +22,14 @@ from .checkpoint_utils import patch_batchnorm
 
 # https://docs.python.org/3/library/threading.html#thread-local-data
 # Manage the checkpoint context with thread-local data.
-class ThreadLocal(threading.local):
-    def __init__(self) -> None:
-        self.is_checkpointing = False
-        self.is_recomputing = False
-        self.is_checkpointing_disabled = False
+@dataclass
+class ThreadLocalCheckpointingState(threading.local):
+    is_checkpointing: bool = False
+    is_recomputing: bool = False
+    is_checkpointing_disabled: bool = False
 
 
-thread_local = ThreadLocal()
+thread_local = ThreadLocalCheckpointingState()
 
 
 @contextmanager