accelerate/data_loader.py: do not yield if the base_dataloader is empty

[0xnightwind]

in the code:

        dataloader_iter = self.base_dataloader.__iter__()
        # We iterate one batch ahead to check when we are at the end
        try:
            current_batch = next(dataloader_iter)
        except StopIteration:
            yield

If the base dataloader is empty then the exception is raised but yield yields nothing.

This at the time of:

if self.device is not None:
                    current_batch = send_to_device(current_batch, self.device, non_blocking=self._non_blocking)

would lead to uncaught exception like:
File "/root/rl-swarm/.venv/lib/python3.10/site-packages/accelerate/data_loader.py", line 575, in iter
current_batch = send_to_device(current_batch, self.device, non_blocking=self._non_blocking)
UnboundLocalError: local variable 'current_batch' referenced before assignment because current_batch was never assigned because next(dataloader_iter) returned with exception StopIteration.

What does this PR do?

Fixes # (issue)

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[0xnightwind]

I'm not sure if I'm tagging the correct folks but picking up @muellerzr @sgugger from the contributors list, apologies in case It wasn't appropriate.

[SunMarc]

Thanks for handling this edge case !

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