[typing] Add typehint for broadcast in training type plugin (#6777)

* Update training_type_plugin.py

* Update accelerator.py

* Update pytorch_lightning/plugins/training_type/training_type_plugin.py

Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

pytorch_lightning/accelerators/accelerator.py

@@ -480,7 +480,7 @@ def connect_precision_plugin(self, plugin: PrecisionPlugin) -> None:
         )
         self.setup_precision_plugin(plugin)
 
-    def save_checkpoint(self, checkpoint: Dict[str, Any], filepath) -> None:
+    def save_checkpoint(self, checkpoint: Dict[str, Any], filepath: str) -> None:
         """Save model/training states as a checkpoint file through state-dump and file-write.
 
         Args:

pytorch_lightning/plugins/training_type/training_type_plugin.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 import contextlib
 from abc import ABC, abstractmethod
-from typing import Any, Callable, Dict, Generator, Iterable, Optional, Tuple, TYPE_CHECKING, Union
+from typing import Any, Callable, Dict, Generator, Iterable, Optional, Tuple, TYPE_CHECKING, TypeVar, Union
 
 import torch
 from torch.nn import Module
@@ -30,6 +30,8 @@
 if TYPE_CHECKING:
     from pytorch_lightning.trainer.trainer import Trainer
 
+TBroadcast = TypeVar("T")
+
 
 class TrainingTypePlugin(Plugin, ABC):
     """A Plugin to change the behaviour of the training, validation and test-loop."""
@@ -88,7 +90,7 @@ def barrier(self, name: Optional[str] = None) -> None:
         """Forces all possibly joined processes to wait for each other"""
 
     @abstractmethod
-    def broadcast(self, obj: object, src: int = 0) -> object:
+    def broadcast(self, obj: TBroadcast, src: int = 0) -> TBroadcast:
         """Broadcasts an object to all processes"""
 
     @abstractmethod