input one, extract multiple across N dim

[nihui]

Mat is a 3-dim data structure. we do not support batch inference on ncnn
however, it can be easier for use if the extractor could extract blob with N dim produced from the network
ex. faster-rcnn, lstm ...

Extractor ex = net.create_extractor();

ncnn::Mat in;
ex.input("data", in);

std::vector<ncnn::Mat> results;
ex.extract("output", results);

[mike07026]

Yes, it will be better if Mat support NCHW format.

[zhedasuiyuan]

What's the status for this issue? Any plans to release this feature? Thanks!