add vllm get_ppl (#1003)

* add vllm get_ppl

* add vllm get_ppl

* format

---------
Co-authored-by: xingjin.wang <xingjin.wang@mihoyo.com>
Co-authored-by: Leymore <zfz-960727@163.com>

opencompass/models/vllm.py

 from typing import Dict, List, Optional
 
+import numpy as np
+
 from opencompass.models.base import BaseModel
 from opencompass.utils import get_logger
 
@@ -103,6 +105,29 @@ class VLLM(BaseModel):
 
         return output_strs
 
+    def get_ppl(self,
+                inputs: List[str],
+                mask_length: Optional[List[int]] = None) -> List[float]:
+        batch_size = len(inputs)
+        sampling_kwargs = SamplingParams(prompt_logprobs=0,
+                                         **self.generation_kwargs)
+        # forward
+        outputs = self.model.generate(inputs, sampling_kwargs)
+        # compute ppl
+        ce_loss = []
+        for i in range(batch_size):
+            prompt_logprobs = outputs[i].prompt_logprobs[1:]
+            prompt_token_ids = outputs[i].prompt_token_ids[1:]
+            prompt_logprobs_list = [
+                prompt_logprobs[i][prompt_token_ids[i]]
+                for i in range(len(prompt_logprobs))
+            ]
+            prompt_logprobs_list = [i.logprob for i in prompt_logprobs_list]
+            prompt_logprobs_list = np.array(prompt_logprobs_list)
+            loss = -prompt_logprobs_list.sum(axis=-1) / len(prompt_token_ids)
+            ce_loss.append(loss)
+        return np.array(ce_loss)
+
     def prompts_preproccess(self, inputs: List[str]):
         if self.use_fastchat_template:
             try: