[BugFix] Fix use of per-request seed with pipeline parallel (#6698)

tests/samplers/test_sampler.py

@@ -510,13 +510,16 @@ def test_sampler_mixed(seed: int, device: str):
             ))
         seq_lens.append(seq_group_metadata_list[-1].seq_data[0].get_len())
 
+    generators: Dict[str, torch.Generator] = {}
+
     def test_sampling():
         sampling_metadata = SamplingMetadata.prepare(
             seq_group_metadata_list,
             seq_lens,
             query_lens=seq_lens,
             device=device,
-            pin_memory=is_pin_memory_available())
+            pin_memory=is_pin_memory_available(),
+            generators=generators)
         sampler_output = sampler(logits=fake_logits,
                                  sampling_metadata=sampling_metadata)