Correct zero division error in inverse sqrt scheduler (#28982)

* Correct zero division error in inverse sqrt scheduler

* default timescale to 10_000

src/transformers/optimization.py

@@ -317,7 +317,7 @@ def get_inverse_sqrt_schedule(
     # https://github.com/google-research/big_vision/blob/f071ce68852d56099437004fd70057597a95f6ef/big_vision/utils.py#L930
 
     if timescale is None:
-        timescale = num_warmup_steps
+        timescale = num_warmup_steps or 10_000
 
     lr_lambda = partial(_get_inverse_sqrt_schedule_lr_lambda, num_warmup_steps=num_warmup_steps, timescale=timescale)
     return LambdaLR(optimizer, lr_lambda, last_epoch=last_epoch)