GradientDescent linesearch parameters

Hi,

I have a couple of questions around the `GradientDescent()` implementation, and I was hoping someone could help me with them:

1. What're the default params for backtracking linesearch within `GradientDescent()` , i.e., when `stepsize <=0`? Are these `init_stepsize=1` and `decrease_factor=0.8`?
2. Is there a way to pass in linesearch params into `GradientDescent()` today? I noticed there's a change underway to allow `stepsize` to be callable - would that take care of this case?

Thanks for the great library - its easy to get good results with the defaults!