Avoid a deprecation warning (#181)

Removes the warning:
`FutureWarning: This implementation of AdamW is deprecated and will be removed in a future version. Use the PyTorch implementation torch.optim.AdamW instead`

finetune.py

@@ -182,6 +182,7 @@ def train(
             learning_rate=learning_rate,
             fp16=True,
             logging_steps=10,
+            optim="adamw_torch",
             evaluation_strategy="steps" if val_set_size > 0 else "no",
             save_strategy="steps",
             eval_steps=200 if val_set_size > 0 else None,