[example] Add diff sph demo (#4769)

* [example] add diff sph demo

* add pretrained models

* add comments and rename the file

Author: erizmr

MANIFEST.in

@@ -6,6 +6,8 @@ include *.cfg
 include python/taichi/*.md
 recursive-include python/taichi/assets *
 recursive-include python/taichi/examples *.py
+include python/taichi/examples/autodiff/diff_sph/fc1_pretrained.pkl
+include python/taichi/examples/autodiff/diff_sph/fc2_pretrained.pkl
 include python/taichi/_lib/core/*.so
 include python/taichi/_lib/core/*.pyd
 include python/taichi/_lib/runtime/*.bc