Allows fpn networks to be recognised by FBNetV2ModelArch

[petoor]

As mentioned in #52
setting cfg.MODEL.FBNET_V2.ARCH = FBNetV3_C_fpn" or any other A-H fpn now works.
Still uses "faster/mask_rcnn_fbnetv3g_fpn.yaml" as cfg in order to tested.
An indivudial yaml file and pretrained model is still needed.

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