debug option for objectnav

[marlohmann]

It would be great if objectnav also had a debug dataset like pointnav does.

generally, I wonder if contributors should be encouraged to implement a debug option if writing a new task. If the task's debug mode is on, only simple, hand-picked, instantiations of the task are sampled for training, and testing is on the same scenes (or very similar scenes, for which generalization should be much easier than in the full setting).