Number of gradient evaluations not applicable to scipy's gradient-free optimizers

[dominikstrb]

Since commit #28a474e, scipy optimizers that do not use gradients do not work from jaxopt.ScipyMinimize anymore.

Because res.njev is accessed on the result regardless of the method argument, you get an

AttributeError: njev. Did you mean: 'nfev'?

if you run jaxopt.ScipyMinimize e.g. with Nelder-Mead or Powell.