Update the (expected) value of the objective function in an `adaQN_free` object, after it has been requested by the optimizer (do NOT update it otherwise).
update_fun(optimizer, fun)
An `adaQN_free` object which after the last run had requested a new function evaluation.
Function as evaluated (in expectation) on the values of `x` that were returned in the request.
No return value (object is updated in-place).