[V1][Spec Decode] Eagle unit tests (#17350)

Signed-off-by: wwl2755 <wangwenlong2755@gmail.com>

vllm/v1/spec_decode/eagle.py

@@ -223,6 +223,8 @@ class EagleProposer:
             hidden_states = hidden_states[:batch_size]
             logits = self.model.compute_logits(last_hidden_states[:batch_size],
                                                None)
+
+            # TODO(wenlong): get more than one token for tree attention
             draft_token_ids = logits.argmax(dim=-1)
             draft_token_ids_list.append(draft_token_ids)
 
@@ -251,6 +253,8 @@ class EagleProposer:
         # [a, b, c] -> [a - n1, b - n2, c - n3]
         num_tokens_per_req = query_len_per_req - num_rejected_tokens
 
+        # [a - n1, b - n2, c - n3] ->
+        # [0, a - n1, a + b - n1 - n2, a + b + c - n1 - n2 - n3]
         cu_num_tokens = torch.empty_like(cu_target_query_lens)
         torch.cumsum(num_tokens_per_req, dim=0, out=cu_num_tokens[1:])
         cu_num_tokens[0] = 0