caffe2/python/task: added __repr__ methods to all task definitions (#15250)

caffe2/python/task: added __repr__ methods to all task definitions (#15250)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15250

This adds `__repr__` methods to all of the classes under task.py. This makes the objects much easier to interact with when using them in an interactive manner, such as in a Jupyter notebook.

The default `__repr__` method just returns the object ID which is very unhelpful.

Reviewed By: hanli0612

Differential Revision: D13475758

fbshipit-source-id: 6e1b166ec35163b9776c797b6a2e0d002560cd29