gluoncv voc_detection

https://github.com/zhreshold/gluon-cv/commit/73b3986aaa2e0d0e6f3f428c12072e3a9d29905e

gluoncv可能版本还没更新，不太小bug已经解决了。

至此，gluoncv 检测部分，从本身的训练集制做，到难样例标记，到评测的坑全踩完了。

        def as_numpy(a):
            """Convert a (list of) mx.NDArray into numpy.ndarray"""
            if isinstance(a, (list, tuple)):
                out = [x.asnumpy() if isinstance(x, mx.nd.NDArray) else x for x in a]
                try:
                    out = np.concatenate(out, axis=0)
                except ValueError:
                    out = np.array(out)
                return out
            elif isinstance(a, mx.nd.NDArray):
                a = a.asnumpy()
            return a