Improve the precision of DirectInfluence with second order mode Gauss Newton

As discussed in #591 it might be possible to build up the inverse of the Gauss Newton matrix directly without materializing the Gauss Newton matrix (as currently done [here](https://github.com/aai-institute/pyDVL/blob/c1ef37bbb682a02f891431989180ea94653fa0aa/src/pydvl/influence/torch/influence_function_model.py#L455)).

On top of a potential speedup, this could increase the precision as it avoids Sin 2 of the [7 sins of numerical linear algebra](https://nhigham.com/2022/10/11/seven-sins-of-numerical-linear-algebra/)