[V0][Bugfix] Fix parallel sampling performance regression when guided decoding is enabled (#17731)

Signed-off-by: Madeesh Kannan <shadeMe@users.noreply.github.com>
Co-authored-by: Russell Bryant <rbryant@redhat.com>

vllm/sequence.py

@@ -1494,7 +1494,7 @@ class ParallelSampleSequenceGroup(SequenceGroupBase):
         for i in range(original_params.n):
             request_id_i = f"{request_id}_parallel_sample_{i}"
             group.seq_id_to_index[request_id_i] = i
-            params = copy.deepcopy(original_params)
+            params = params.clone()
             params.n = 1
             if params.seed is not None:
                 params.seed += i