text: Title: [Bug] sgl.Engine(**dataclasses.asdict(server_args)) return_logprob=True error
  Body: ### Checklist
  
  - [x] 1. I have searched related issues but cannot get the expected help.
  - [ ] 2. The bug has not been fixed in the latest version.
  - [ ] 3. Please note that if the bug-related issue you submitted lacks corresponding environment info and a minimal reproducible demo, it will be challenging for us to reproduce and resolve the issue, reducing the likelihood of receiving feedback.
  - [ ] 4. If the issue you raised is not a bug but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
  - [ ] 5. Please use English, otherwise it will be closed.
  
  ### Describe the bug
  
  I use Sglang Engine to run offfine inference with qwen2-vl-2b, and I want to obtain the  logprob of generated tokens, but when I run the code 
  sampling_params = {"temperature": 0.8, "top_p": 0.95,  "stop": ["<|endoftext|>", "</s>"], "max_new_tokens": 1}
  
  `vlm = sgl.Engine(**dataclasses.asdict(server_args))
          outputs = await vlm.async_generate(prompts, sampling_params, return_logprob=[True]*len(prompts),
              image_data=image_datas)`
  
  and 
  `vlm = sgl.Engine(**dataclasses.asdict(server_args))
          outputs = await vlm.async_generate(prompts, sampling_params, return_logprob=True,
              image_data=image_datas)`
  
  the errors  are the same as 
  `
  [2025-03-05 15:02:47 TP0] max_total_num_tokens=2303336, chunked_prefill_size=-1, max_prefill_tokens=16384, max_running_requests=4097, context_len=32768
  [2025-03-05 15:02:52 TP0] Prefill batch. #new-seq: 1, #new-token: 282, #cached-token: 0, cache hit rate: 0.00%, token usage: 0.00, #running-req: 0, #queue-req: 0
  [2025-03-05 15:02:52 TP0] Scheduler hit an exception: Traceback (most recent call last):
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/managers/scheduler.py", line 1827, in run_scheduler_process
      scheduler.event_loop_normal()
    File "/home/shuxin/.local/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 116, in decorate_context
      return func(*args, **kwargs)
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/managers/scheduler.py", line 478, in event_loop_normal
      result = self.run_batch(batch)
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/managers/scheduler.py", line 1080, in run_batch
      logits_output, next_token_ids = self.tp_worker.forward_batch_generation(
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/managers/tp_worker.py", line 164, in forward_batch_generation
      logits_output = self.model_runner.forward(forward_batch)
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/model_executor/model_runner.py", line 796, in forward
      return self.forward_extend(forward_batch)
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/model_executor/model_runner.py", line 761, in forward_extend
      return self.model.forward(
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/models/qwen2_vl.py", line 571, in forward
      return self.logits_processor(
    File "/home/shuxin/.local/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1739, in _wrapped_call_impl
      return self._call_impl(*args, **kwargs)
    File "/home/shuxin/.local/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1750, in _call_impl
      return forward_call(*args, **kwargs)
    File "/home/shuxin/miniconda3/envs/qwen-vl/lib/python3.10/site-packages/sglang/srt/layers/logits_processor.py", line 164, in forward
      pruned_input_ids.append(input_ids[pt + start_len : pt + extend_len])
  TypeError: 'NoneType' object is not subscriptable`
  
  [2025-03-05 15:02:52] Received sigquit from a child proces. It usually means the child failed.
  
  where is my set incorrect?
  
  
  ### Reproduction
  
  qwen2-vl-2b
  
  ### Environment
  
  linux 
  PRETTY_NAME="Ubuntu 22.04.4 LTS"
  NAME="Ubuntu"
  VERSION_ID="22.04"
  VERSION="22.04.4 LTS (Jammy Jellyfish)"
  VERSION_CODENAME=jammy
  ID=ubuntu
  ID_LIKE=debian
  HOME_URL="https://www.ubuntu.com/"
  SUPPORT_URL="https://help.ubuntu.com/"
  BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
  
  sglang-0.4.3.post2