[Example] Add cg_possion.py, a Poisson solver based on conjugate gradients (#1595)
[Example] Add cg_possion.py, a Poisson solver based on conjugate gradients (#1595)

* [Example] Add cg_possion.py, a Possion Solver based on Conjugate Gradient

* [skip ci] enforce code format

* [skip ci] apply

Co-authored-by: Taichi Gardener <52844ac7498fba84f633481a7c3ae54491811c81@gmail.com>
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi