Implement ACFB

[azhx]

Where is the code for this?
Implementation will wait until finetuning with current RAD-Net is done.

[azhx]

1. No, the code for this is acknowledged to not exist
2. I propose not implementing this at all, because details are lacking and dynamic box priors likely try to solve the same problem that ACFB claims to solve.
   https://github.com/WATonomous/action-classification/wiki/Actor-Context-Feature-Bank-(ACFB)