[BugFix][Spec Decode] No in-place update to draft probs (vllm-project#16952)

Signed-off-by: Woosuk Kwon <[email protected]>

Author: WoosukKwon

vllm/v1/spec_decode/eagle.py

@@ -264,7 +264,9 @@ def compute_probs_and_sample_next_token(
     # TODO(woosuk): Consider seeds.
     q = torch.empty_like(probs)
     q.exponential_()
-    next_token_ids = probs.div_(q).argmax(dim=-1).view(-1)
+    # NOTE(woosuk): We shouldn't use `probs.div_(q)` because the draft_probs
+    # will be used later for rejection sampling.
+    next_token_ids = probs.div(q).argmax(dim=-1).view(-1)
     if not sampling_metadata.all_random:
         greedy_token_ids = probs.argmax(dim=-1)
         next_token_ids = torch.where(