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.

[facebook-github-bot]

Hi @petoor!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@fb.com. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

@zhanghang1989 has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@petoor has updated the pull request. You must reimport the pull request before landing.