[RFC][caffe2] TaskGroup.__repr__ shouldn't have side effects
Summary: `__repr__` calling self.tasks() ends up marking the instance as "used", which doesn't seem appropriate. I was debugging a value being passed around and then ran into `Cannot add Task to an already used TaskGroup.` because the value had been logged once.
Test Plan:
Added a unit test -- didn't see a clean public method to test it, but I'm happy to add one if that makes sense.
Will wait for sandcastle to trigger everything else; I'm not at all familiar with this code so any other recommendations would be great!
Reviewed By: cryptopic
Differential Revision: D23541198
fbshipit-source-id: 5d1ec674a1ddaedf113140133b90e0da6afa7270
