feat: extract logprobs from dict messages for GRPO importance sampling

Author: JRMeyer

src/art/preprocessing/tokenize.py

@@ -221,7 +221,15 @@ def tokenize_trajectory(
                 add_special_tokens=False,
             )
             token_ids[start:end] = content_token_ids
-            logprobs[start:end] = [float("nan")] * len(content_token_ids)
+            dict_logprobs = message.get("logprobs")
+            if dict_logprobs is None:
+                logprobs[start:end] = [float("nan")] * len(content_token_ids)
+            elif "content" in dict_logprobs and dict_logprobs["content"]:
+                logprobs[start:end] = [lp["logprob"] for lp in dict_logprobs["content"]]
+            else:
+                raise ValueError(
+                    f"Message has 'logprobs' key but content is missing or empty: {dict_logprobs}"
+                )
             assistant_mask[start:end] = [1] * len(content_token_ids)
         else:
             choice = message