[Example] Improve stable fluids (#2361)
[Example] Improve stable fluids (#2361)

* toggle modes, add gravity

* [skip ci] enforce code format

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