[feature][extendrt] experimental parallel compile graph · Pull Request !317 · MindSpore/mindspore-lite

1. feat: experimental parallel compile graph
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**What type of PR is this?**

/kind feature

**What does this PR do / why do we need it**:
当前云侧多线程实例在加载模型时为串行，这个PR提供了实验性的配置，可使多线程实例并行加载。
配置文件新增`experimental` section，`compile_graph_parallel`配置项。
`compile_graph_parallel`可选值为：
|  值 |  说明 |
|---|---|
| off  |  关闭，默认值 |
| on  |  开启 |

当用户设置其他值时告警提示正确配置方法，并采用默认配置。

**配置示例**：
开启实验性并行编译:
```ini
[experimental]
compile_graph_parallel=on
```

关闭实验性并行编译:
```ini
[experimental]
compile_graph_parallel=off
```

**实现方案：**
在编译函数`GraphSinkSession::CompileGraph`中，根据用户配置在合适位置手动释放锁。

```cpp
std::unique_lock<std::mutex> lock(g_build_graph_mutex);  // 可在生命周期结束自动释放锁

// 临界区
// ...

// 手动释放
if (CheckCompileGraphParallel(GetConfigOption(lite::kExperimentalSection, lite::kExperimentalSection))) {
  lock.unlock();
}
```

**Which issue(s) this PR fixes**:

Fixes #ICZBID

**Code review checklist [【代码检视checklist说明】](https://gitee.com/mindspore/community/blob/master/security/code_review_checklist_mechanism.md)**:

+ - [ ] 是否进行返回值校验 (禁止使用void屏蔽安全函数、自研函数返回值，C++标准库函数确认无问题可以屏蔽)
+ - [ ] 是否遵守 ***SOLID原则 / 迪米特法则***
+ - [ ] 是否具备UT测试用例看护 && 测试用例为有效用例 (若新特性无测试用例看护请说明原因)
+ - [ ] 是否为对外接口变更
+ - [ ] 是否涉及官网文档修改

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