[ddp] fixing data slice bug (vllm-project#2361)

Summary:

that's not how you slice a dataset, previously not tested with
world_size==1

Test Plan:

[script](https://gist.github.com/HDCharles/282950166fd0c95a7a2594fe922bcb53)

(world_size==1)

---------

Signed-off-by: HDCharles <charlesdavidhernandez@gmail.com>
Signed-off-by: HDCharles <39544797+HDCharles@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Signed-off-by: 김대익 <33992354+dik654@users.noreply.github.com>

Author: 2 people

src/llmcompressor/datasets/utils.py

@@ -260,7 +260,7 @@ def _make_sampler(args: DatasetArguments, dataset: Dataset) -> Sampler:
         start, end = _get_partition_start_end(
             num_samples, dist.get_rank(), dist.get_world_size()
         )
-        dataset = dataset[start:end]
+        dataset = dataset.select(range(start, end))
 
     if num_samples is not None and num_samples > len(dataset):
         logger.warning(