[Bugfix] Fix speculative decode seeded test

[tdoublep]

I was looking into some issues that @njhill was facing with #6449 and realized that the test to verify per-request seeding for spec decode is not functioning correctly. This is because currently the baseline_llm_generator and the test_llm_generator are setting the same global seed.

In order to ensure that the test cover the desired functionality, we need to allow for the baseline and test to get seeded differently, which is what is implemented in this PR.

I have verified that applying either of the following two changes would now cause the test to fail:

diff --git a/vllm/spec_decode/spec_decode_worker.py b/vllm/spec_decode/spec_decode_worker.py
index 8cf0aa5b..17e54a31 100644
--- a/vllm/spec_decode/spec_decode_worker.py
+++ b/vllm/spec_decode/spec_decode_worker.py
@@ -598,11 +598,14 @@ class SpecDecodeWorker(LoraNotSupportedWorkerBase):
             # Get sequence group state
             generators = []
             for seq_group_metadata in seq_group_metadata_list:
+                '''
                 if (seq_group_metadata.state is not None
                         and seq_group_metadata.state.generator is not None):
                     generators.append(seq_group_metadata.state.generator)
                 else:
                     generators.append(None)
+                '''
+                generators.append(None)
 
             sampler_extra_kwargs["generators"] = generators

or

diff --git a/vllm/spec_decode/batch_expansion.py b/vllm/spec_decode/batch_expansion.py
index 41f0aebf..df160134 100644
--- a/vllm/spec_decode/batch_expansion.py
+++ b/vllm/spec_decode/batch_expansion.py
@@ -293,6 +293,7 @@ class BatchExpansionTop1Scorer(SpeculativeScorer):
         for data in new_seq_data_dict.values():
             data.update_num_computed_tokens(data.get_len() - 1)
 
+        '''
         if (seq_group_metadata.state is not None
                 and seq_group_metadata.state.generator is not None):
             generator = torch.Generator(
@@ -301,6 +302,8 @@ class BatchExpansionTop1Scorer(SpeculativeScorer):
             state = SequenceGroupState(generator=generator)
         else:
             state = None
+        '''
+        state = None

[github-actions]

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[tdoublep]

Closing as this will be addressed by #6743