handle predict kwargs in historical_forecasts() and backtest()

[hrzn]

No description provided.

[RayDanks]

Hi, is this still outstanding with no assignee ? If so I'd like to take it as my first issue. I've already had a look through and familiarised myself with this particular issue, but will describe a bit more here once confirmed to be sure on my approach since it's my first contrib to this repo. Thanks :)

[hrzn]

Hi @RayDanks and sorry for the late answer!
I think this should constitute a fairly good first issue to work on. Basically we need to accept predict_kwargs arguments in historical_forecasts() and backtest() and propagate them all the way to the calls to predict().

[madtoinou]

Implemented in #2050.