支持选择模型下的多个文件

1、新建云脑任务时，支持选择同一个模型下面的多个文件。
2、创建云脑任务时的样例代码也需要提供支持多个文件的样例。 @liuzx
3、先支持智算NPU、启智NPU的训练、推理云脑任务。调试等重构完成后再支持多模型文件。
4、帮助文档需要同步更新。 @chenzh

若选择了同一个模型下的多个文件，模型引用数只加1。

需要基于新版的示例代码进行测试，代码分支https://openi.pcl.ac.cn/OpenIOSSG/MNIST_Example/src/branch/V20230427

调试任务都没做，包括启智NPU、智算NPU。

通过测试，已经上线。

Deleting a branch is permanent. It CANNOT be undone. Continue?

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#4014 支持选择模型下的多个文件