Fix logprob calculation. Fixes ggml-org#134

Author: abetlen

llama_cpp/llama.py

@@ -638,7 +638,7 @@ def _create_completion(
                 for token in all_tokens
             ]
             all_logprobs = [
-                [Llama.logit_to_logprob(logit) for logit in row]
+                Llama._logits_to_logprobs(row)
                 for row in self.eval_logits
             ]
             for token, token_str, logprobs_token in zip(
@@ -980,5 +980,7 @@ def token_bos() -> llama_cpp.llama_token:
         return llama_cpp.llama_token_bos()
 
     @staticmethod
-    def logit_to_logprob(x: float) -> float:
-        return math.log(1.0 + math.exp(x))
+    def logits_to_logprobs(logits: List[llama_cpp.c_float]) -> List[llama_cpp.c_float]:
+        exps = [math.exp(float(x)) for x in logits]
+        sum_exps = sum(exps)
+        return [llama_cpp.c_float(math.log(x / sum_exps)) for x in exps]