[Doc] Add Python docstring for SNode

[k-ye]

Related issue = #2579

In addition to the docstring itself, this PR serves as an example for how to do class/method referencing.

[k-ye]

/format

[rexwangcc]

I left a few comments but in general looks good!

One minor question: is the reason we don't use type annotations in python/taichi/lang/snode.py that it requires extra import List, ... from typing, or that it simply hasn't happened yet?

[k-ye]

One minor question: is the reason we don't use type annotations in python/taichi/lang/snode.py that it requires extra import List, ... from typing, or that it simply hasn't happened yet?

It's a combination of me being lazy and the code hierarchy: I was planning to use the Axis type, but then ran into circular import issue...

[rexwangcc]

One minor question: is the reason we don't use type annotations in python/taichi/lang/snode.py that it requires extra import List, ... from typing, or that it simply hasn't happened yet?

It's a combination of me being lazy and the code hierarchy: I was planning to use the Axis type, but then ran into circular import issue...

Ah, gotcha. I'm fine with leaving them as is then! (we could use forward references to bypass the circular import, but I guess we can solve it once and for all by refactoring the code later?)

[rexwangcc]

LGTM

[k-ye]

(we could use forward references to bypass the circular import, but I guess we can solve it once and for all by refactoring the code later?)

+1, I feel like the code needs some refactor before we have all the types we need for annotation