[Lang] Make @ti.func callable from Python-scope, or @ti.kfunc if we're afraid of breaking API

[archibate]

Concisely describe the proposed feature
It would be convenient to unify these two concepts, both for API clarity and reducing user learning obstacle.
This issue will be just a begining of this refactor.

Describe the solution you'd like (if any)
We may first add @ti.kfunc as a experimental one, then replace @ti.func when it's mature and battle-tested.

1. When @ti.func called from Python-scope will be translated into a minimal kernel that executes it, and exit.
2. Add @ti.kfunc that create both a @ti.func and @ti.kernel, when called from corresponding scope, call func/kernel.

def kfunc(foo):
  func_foo = ti.func(foo)
  kernel_foo = ti.kernel(foo)
  @functools.wraps(foo)
  def wrapped(*args, **kwargs):
    if impl.inside_kernel():
      func_foo(*args, **kwargs)
    else:
      kernel_foo(*args, **kwargs)

Additional comments
@yuanming-hu @k-ye WDYT? Inputs are welcome!

[archibate]

I managed to make ti.kfunc, but I failed to nake it work for ODOP, @k-ye could you share some solutions?