Update the (expected) gradient in an optimizer from this package, after it has been requested by the optimizer (do NOT update it otherwise).
update_gradient(optimizer, gradient)
A free-mode optimizer from this package (`oLBFGS_free`, `SQN_free`, `adaQN_free`) which after the last run had requested a new gradient evaluation..
The (expected value of the) gradient as evaluated on the values of `x` that were returned in the request. Must be a numeric vector.
No return value (object is updated in-place).