[Example] Add Multiple Importance Sampling to the cornell box scene (#890)
[Example] Add Multiple Importance Sampling to the cornell box scene (#890)

* [Example] Add Multiple Importance Sampling to the cornell box scene

* [skip ci] nits

* fix samples per second

Co-authored-by: Yuanming Hu <8b3703cd4f53131067509a7ec456fe662694d257@gmail.com>
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi