Resetting differentiation input in-between differentiations

[blegat]

In https://discourse.julialang.org/t/differentiating-through-a-qp-order-of-constraints/119275/3, it is argued that the input of the differentiation should be reset in-between different calls to DiffOpt.forward_differentiate!(model).
We should clarify this in the documentation.

[joaquimg]

#262 solves this by adding: empty_input_sensitivities!