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

* support 8bit optims from bnb.

* fix exist_ok

* use_8bit_bnb.

* fix.

* note in readme.

finetrainers/utils/optimizer_utils.py

@@ -39,6 +39,7 @@ def get_optimizer(
             weight_decay=weight_decay,
         )
 
+    # TODO: consider moving the validation logic to `args.py` when we have torchao.
     if use_8bit and use_4bit:
         raise ValueError("Cannot set both `use_8bit` and `use_4bit` to True.")
 
@@ -46,7 +47,6 @@ def get_optimizer(
         try:
             import torchao
 
-            torchao.__version__
         except ImportError:
             raise ImportError(
                 "To use optimizers from torchao, please install the torchao library: `USE_CPP=0 pip install torchao`."