MegEngine 无法正常启用 CUDA 时应该如何解决

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在使用 MegEngine 时，MegEngine 会自动选择最快的计算设备（即 XPU），在 CUDA 不可用时，您可能会遇到如下几种文字提示您当前 MegEngine 运行在 CPU 模式下：

info: +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
info: + Failed to load CUDA driver library, MegEngine works under CPU mode now.         +
info: + To use CUDA mode, please make sure NVIDIA GPU driver was installed properly.    +
info: + Refer to https://discuss.megengine.org.cn/t/topic/1264 for more information.    +
info: +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

02 11:47:53[mgb] WRN cuda unavailable: no CUDA-capable device is detected(100) ndev=-1

err: Failed to load cuda API library
err: failed to load cuda func: cuCtxGetCurrent
err: failed to load cuda func: cuDeviceGetCount
err: failed to load cuda func: cuGetErrorString
02 09:10:10[mgb] WRN cuda unavailable: unknown cuda error(999) ndev=-1

如果您确定已经正确安装了 NVIDIA 显卡设备，却仍然无法正常启用 CUDA 模式，请检查如下问题：

1. 是否正常安装了显卡驱动并识别到显卡？请通过 nvidia-smi 确认显卡状态
2. 驱动版本是否过老？
3. 是否设置了 CUDA_VISIABLE_DEVICES 等环境变量屏蔽了全部显卡设备？
4. 当前是否在 docker 环境中？如是，是否正确通过 nvidia-docker 把对应设备和 lib 映射到正确位置？