SNodeType=pointer not supported on Metal

[gdwSH]

Running the codes on macOS:
https://forum.taichi.graphics/t/homework1-eulerian-fluid-simulation-smoke/997/2

Internal Error occurred, check this page for possible solutions:
https://taichi.readthedocs.io/en/stable/install.html#troubleshooting
Traceback (most recent call last):
File "smoke.py", line 577, in
main()
File "smoke.py", line 573, in main
smoke_solver.run()
File "smoke.py", line 543, in run
self.reset()
File "smoke.py", line 521, in reset
self._dens_buffer.fill(0.0)
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/util.py", line 212, in wrapped
return func(*args, **kwargs)
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/expr.py", line 126, in fill
fill_tensor(self, val)
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/kernel.py", line 580, in wrapped
return primal(*args, **kwargs)
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/kernel.py", line 507, in call
self.materialize(key=key, args=args, arg_features=arg_features)
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/kernel.py", line 309, in materialize
self.runtime.materialize()
File "/Users/guodewen/anaconda3/envs/taichi/lib/python3.7/site-packages/taichi/lang/impl.py", line 216, in materialize
taichi_lang_core.layout(layout)
RuntimeError: [struct_metal.cpp:generate_types@230] SNodeType=pointer not supported on Metal

Running time: 0.67s

[archibate]

This is because the sparse feature is not yet supported on Metal backend.
Solution:

1. Replace ti.init(ti.gpu) by ti.init(ti.cpu).
2. Replace all xxx.pointer by xxx.bitmasked.

[k-ye]

This feature is finally shipped in https://github.com/taichi-dev/taichi/releases/tag/v0.7.25. Feel free to give it a try, thx!