use IPS estimator for average loss of --cb_type dr

[marco-rossi29]

See #1696 for more details

--cb_type dr is a mode of learning, but we should enable the ability to use the IPS estimator to compute the average loss

[maxpagels]

Working on OPE docs for vowpalwabbit.org (https://github.com/VowpalWabbit/vowpalwabbit.github.io/pull/193), and I don't know the state of this issue, but allowing learning using some cb_type and reporting loss using something else like IPS would be very useful. I suspect many people just gridsearch everything and wonder why it isn't working.

[rangi513]

Should this feature request also include the ability to use the IPS/DR estimator for evaluating the average PV loss of a policy (offline) using --cb_type mtr in the learning algorithm?

[jackgerrits]

To address this we can:

• [ ] Port Python implementations of estimators to C++ (separate lib to core)
• [ ] Change loss calculation for CB explore reductions to use estimators
• [ ] Make the choice of estimator configurable