Fix Resnet-101 model in example of MXNET

SSD(Single Shot MultiBox Detector) is the fastest method in object-detection task (Another detector YOLO, is a little bit slower than SSD). In the source code of MXNET,there is an example for SSD implementation. I test it by using different models: inceptionv3, resnet-50, resnet-101 etc. and find a weird phenomenon: the size .params file generated by resnet-101 is smaller than resnet-50.

Model Size of .params file
resnet-50 119MB
resnet-101 69MB

Since deeper network have larger number of parameters, resnet-101 has smaller file size for parameters seems suspicious.

Reviewing the code of example/ssd/symbol/

Why resnet-50 and resnet-101 has the same ‘from_layers’ ? Let’s check these two models:

In resnet-50, the SSD use two layers (as show in red line) to extract features. One from output of stage-3, another from output of stage-4. In resnet-101, it should be the same (as show in blue line), but it incorrectly copy the config code of resnet-50. The correct ‘from_layers’ for resnet-100 is:

This seems like a bug, so I create a pull request to try fixing it.

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