[optimization] support 8bit optims from bistandbytes (#163)

* support 8bit optims from bnb.

* fix exist_ok

* use_8bit_bnb.

* fix.

* note in readme.

finetrainers/args.py

@@ -79,6 +79,7 @@ class Args:
 
     # Optimizer arguments
     optimizer: str = "adamw"
+    use_8bit_bnb: bool = False
     lr: float = 1e-4
     scale_lr: bool = False
     lr_scheduler: str = "cosine_with_restarts"
@@ -166,6 +167,7 @@ class Args:
             },
             "optimizer_arguments": {
                 "optimizer": self.optimizer,
+                "use_8bit_bnb": self.use_8bit_bnb,
                 "lr": self.lr,
                 "scale_lr": self.scale_lr,
                 "lr_scheduler": self.lr_scheduler,
@@ -532,6 +534,11 @@ def _add_optimizer_arguments(parser: argparse.ArgumentParser) -> None:
         choices=["adam", "adamw"],
         help=("The optimizer type to use."),
     )
+    parser.add_argument(
+        "--use_8bit_bnb",
+        action="store_true",
+        help=("Whether to use 8bit variant of the `--optimizer` using `bitsandbytes`."),
+    )
     parser.add_argument(
         "--beta1",
         type=float,
@@ -741,6 +748,7 @@ def _map_to_args_type(args: Dict[str, Any]) -> Args:
 
     # Optimizer arguments
     result_args.optimizer = args.optimizer or "adamw"
+    result_args.use_8bit_bnb = args.use_8bit_bnb
     result_args.lr = args.lr or 1e-4
     result_args.scale_lr = args.scale_lr
     result_args.lr_scheduler = args.lr_scheduler