fix: 规范 generate.py 命令行接口

- generate.py 清理描述文字，help 统一标注默认值
- max_tokens 默认改为 None，回退 model config max_len
- evaluate_ppl.py 同步清理描述文字
- params.md 同步 max_tokens 默认值

assets/docs/params.md

@@ -161,7 +161,7 @@ See [Inference Guide](inference.md) for HTTP API documentation.
 | `--top_k` | int | `30` | Top-k filtering |
 | `--top_p` | float | `0.95` | Nucleus sampling threshold |
 | `--batch_size` | int | `1` | Batch size for generation |
-| `--max_tokens` | int | `2048` | Maximum tokens to generate |
+| `--max_tokens` | int | model config `max_len` | Maximum tokens to generate |
 
 Usage:
 ```bash