Add AdamW to CPUOffloadOptimizer default (#742)

add default

torchao/prototype/low_bit_optim/cpu_offload.py

@@ -1,20 +1,33 @@
 from typing import Type
 
 import torch
-from torch.optim.optimizer import Optimizer
+from torch.optim.optimizer import Optimizer, ParamsT
+
+from torchao.utils import TORCH_VERSION_AT_LEAST_2_4
 
 
 class CPUOffloadOptimizer:
-    def __init__(self, params, optimizer_class: Type[Optimizer], *, offload_gradients: bool = False, **kwargs) -> None:
+    def __init__(
+        self,
+        params: ParamsT,
+        optimizer_class: Type[Optimizer] = torch.optim.AdamW,
+        *,
+        offload_gradients: bool = False,
+        **kwargs,
+    ) -> None:
         """Offload optimizer to CPU for single-GPU training. This will reduce GPU memory by the size of optimizer state.
         Optimizer step will be done on CPU.
 
         Args
             params: a list of parameters or parameter groups.
-            optimizer_class: constructor of the base optimizer.
+            optimizer_class: constructor of the base optimizer. Defaults to :class:`torch.optim.AdamW`.
             offload_gradients: free GPU gradients once they are moved to CPU. Not compatible with gradient accumulation.
             kwargs: other keyword arguments to be passed to the base optimizer e.g. `lr`, `weight_decay`.
         """
+        # default to fused CPU AdamW
+        if optimizer_class is torch.optim.AdamW and TORCH_VERSION_AT_LEAST_2_4 and "fused" not in kwargs:
+            kwargs.update(fused=True)
+
         param_groups = list(params)
         if len(param_groups) == 0:
             raise ValueError("optimizer got an empty parameter list")