fix: 修复文档多处不准确 + inference scheduler 越界 bug + SchedulerCallback 回调阶段修正

文档 (6 个文件):
- design.md: 15+ 处修正 — persistent_key_values→paged_cache,
  MLA 字段重写, Server/ParallelSetup 不存在类移除,
  关系箭头方向修复, SchedulerCallback 阶段修正等
- dataflow.md: 重写数据流图和描述, 修复训练回调顺序、
  数据键名、MLA 归属、MetricTracker 等错误
- introduction.md: 层数 32→24, MLP 图双 Linear 修正,
  默认值/响应字段/health 端点修复
- params.md: 补充 grpo 及 4 个 GRPO 参数
- README.md / README-zh-CN.md: generate.py 补全必需参数,
  删除重复注释, HuggingFace 声明修正

代码 (2 个文件):
- scheduler.py: n_pages 池加 page_size 余量防止越界;
  decode 前预分配页
- train_callback.py: SchedulerCallback 从 on_step_end 改
  回 on_batch_end (按 batch 步进学习率)

assets/docs/params.md

@@ -6,7 +6,7 @@
 
 | Parameter | Description | Default |
 |-----------|-------------|---------|
-| `--train_type` | Training type (`seq`, `sft`, `dpo`) | required |
+| `--train_type` | Training type (`seq`, `sft`, `dpo`, `grpo`) | required |
 | `--data_root_path` | Dataset root directory | required |
 | `--param_path` | Model parameters or checkpoint path | required |
 | `--n_epoch` | Total training epochs | 1 |
@@ -61,6 +61,10 @@
 |-----------|-------------|---------|---------|
 | `--dpo_beta` | DPO beta value | 0.1 | `dpo` |
 | `--label_smoothing` | Label smoothing for cross-entropy loss | 0.1 | `seq`, `sft` |
+| `--group_size` | GRPO group size | 4 | `grpo` |
+| `--grpo_clip_eps` | GRPO clipping epsilon | 0.2 | `grpo` |
+| `--grpo_kl_coef` | GRPO KL penalty coefficient | 0.01 | `grpo` |
+| `--grpo_sync_interval` | GRPO ref_model sync interval (steps) | 200 | `grpo` |
 
 ### Usage Example