Add FPN head

[quocnhat]

Hi, Thanks for your sharing an awesome work, This is not an issue, just like a question, please help: I would like to add FPN head to your mnetv3 backbone. I see that FPN requires the consecutive double size of the return layers ( 32 -> 64 -> 128...). But the size of the output layers of mobilenetv3 are 16 ->24 ->40 ,... which do not adopt the rule. Do you have any comments? anyway, if modify the net, I can successfully build but with worse accuracy. Thank you.