fix : 修复存储层 bug，JSON 切换为 JSONL，补齐测试覆盖

- save_bin/load_bin: save_json/load_json 替换为直接 json.dump/json.load，修复致命 bug
- _normalize: 空 cum 列表 guard，防止 IndexError
- load_json: 改为仅支持 JSONL 逐行解析 (json.loads)，移除 .json 支持
- detect_format: 只匹配 *.jsonl，不再匹配 *.json
- save_json: 输出扩展名改为 .jsonl
- GRPODataset.__getitem__: 补齐 .to(dtype=torch.long/bool) 与其他数据集一致
- load_bin: np.memmap mode='r+' 消除 PyTorch 不可写 tensor 警告
- 新增 16 个测试: bin roundtrip, mmap load, 空 key, JSONL 多行/文本, GRPO dtype/load, detect_format bin/jsonl, fetch multi-key/越界, json_to_bin 转换, DPO from JSONL, 显式 storage_type

astrai/dataset/dataset.py

@@ -306,9 +306,11 @@ class GRPODataset(BaseDataset):
     def __getitem__(self, index: int) -> Dict[str, Tensor]:
         begin_idx, end_idx = self.get_index(index)
 
-        prompts = self._fetch_data(begin_idx, end_idx, "prompts")
-        responses = self._fetch_data(begin_idx, end_idx, "responses")
-        masks = self._fetch_data(begin_idx, end_idx, "masks")
+        prompts = self._fetch_data(begin_idx, end_idx, "prompts").to(dtype=torch.long)
+        responses = self._fetch_data(begin_idx, end_idx, "responses").to(
+            dtype=torch.long
+        )
+        masks = self._fetch_data(begin_idx, end_idx, "masks").to(dtype=torch.bool)
         rewards = self._fetch_data(begin_idx, end_idx, "rewards")
 
         return {