SSD examples based on MindCV

[DexterJZ]

Implement SSD object detector based on MindCV backbones (MobileNetV2, ResNet50, MobileNetV3)

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Related Issues and PRs

Issue #4

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[geniuspatrick]

1. Callback可以复用仓里已有的吗？
2. 数据集是和模型绑定的还是和数据集绑定的？如果之和coco相关，是不是可以抽象到包内，以便后续检测模型同样能够使用。
3. 文件不必以ssd作为前缀
4. 考虑讲一部分逻辑进行解耦，只将与模型强相关的，放到example里，其他的可以放在包里，以作后续复用

[DexterJZ]

Files renamed. Other tasks are suggested to be marked and left for the future.