Add streaming eval sample logging to BaseEnv

Introduces `log_eval_sample()` method for stream-writing individual
evaluation samples to `samples.jsonl` during evaluation, with lazy
writer initialization and automatic HTML generation on completion.
Updates GSM8k environment to use streaming approach instead of batching
samples.

atroposlib/envs/base.py

@@ -798,6 +798,23 @@ class BaseEnv(ABC):
             except Exception as e:
                 logger.warning("Failed to generate eval HTML viewer: %s", e)
 
+    def log_eval_sample(self, sample):
+        """Stream-write a single eval sample to samples.jsonl.
+
+        Lazy-initializes the writer on first call. Use this inside evaluate()
+        to write samples as they complete rather than batching at the end.
+        If using this, omit the samples= parameter from evaluate_log().
+        """
+        if self._eval_sample_writer is None:
+            if self.config.data_dir_to_save_evals is None:
+                return
+            os.makedirs(self.config.data_dir_to_save_evals, exist_ok=True)
+            self._eval_samples_path = os.path.join(
+                self.config.data_dir_to_save_evals, "samples.jsonl"
+            )
+            self._eval_sample_writer = jsonlines.open(self._eval_samples_path, "w")
+        self._eval_sample_writer.write(sample)
+
     @retry(
         stop=stop_after_attempt(3),
         wait=wait_random_exponential(multiplier=1, max=10),
@@ -1343,10 +1360,23 @@ class BaseEnv(ABC):
         """
         Internal method to run evaluation with proper setup.
         """
+        self._eval_sample_writer = None
+        self._eval_samples_path = None
         await self.setup()
         try:
             await self.evaluate()
         finally:
+            # Close streaming eval sample writer if it was used
+            if self._eval_sample_writer is not None:
+                self._eval_sample_writer.close()
+                if self._eval_samples_path:
+                    try:
+                        from atroposlib.frontend.jsonl2html import generate_eval_html
+
+                        generate_eval_html(self._eval_samples_path)
+                    except Exception as e:
+                        logger.warning("Failed to generate eval HTML: %s", e)
+            # Close JSONL trajectory writer if it was used
             if self.jsonl_writer is not None:
                 self.jsonl_writer.close()
                 if self.config.data_path_to_save_groups: