add feature map 38x38

[OMG59E]

Hi: i want to add feature map 38x38 and remove 2x2. Is it feasible？

[ujsyehao]

@OMG59E the origin SSD has 38x38 feature map.

[xizi]

origin VGG-SSD 有3838的feature map并用作检测，但MobileNet-SSD里面没有把3838的feature map用于检测。