GPGPU训练任务需要完善示例代码

目前GPGPU推理任务有示例代码，但GPGPU训练任务无示例代码

已增加示例代码，测试通过后再合到示例的master分支：https://openi.pcl.ac.cn/OpenIOSSG/OpenI_Cloudbrain_Example/src/branch/liuzx/gpgpu_mnist_example/train.py

2024.3.1 示例代码测试通过

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#5230 GPGPU训练任务需要完善示例代码