don't sort multipack sampler (#2657)

* don't sort multipack sampler

* increased packing efficiency increases loss

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Co-authored-by: Wing Lian <wing@axolotl.ai>

tests/test_packed_batch_sampler.py

@@ -106,3 +106,4 @@ class TestBatchedSamplerPacking:
 
         original_idxs = set(range(len(train_dataset)))
         assert original_idxs == set(batch_idxs)
+        assert len(batch_idxs) == len(set(batch_idxs))