[WIP] Use sparse `grid_AO` in `exchange_correlation(...)`

[blazejba]

In relation to https://github.com/graphcore-research/pyscf-ipu/issues/88

As for now, it has been experimentally done in a separate file on the CPU backend.

The following steps are:

• to figure out why the IPU backend fails
• to integrate the code with pmap in nanoDFT.py