[Example] Fix the ti example problems of sparse matrix demos

[Hanke98]

Related issue = #3216 #3116 #3081

When you type ti example stable_fluid, it will not run the example successfully:

[Taichi] version 0.8.3, llvm 10.0.0, commit 306cdbf3, linux, python 3.9.5

*******************************************
**      Taichi Programming Language      **
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Docs:   https://taichi.rtfd.io/en/stable
GitHub: https://github.com/taichi-dev/taichi
Forum:  https://forum.taichi.graphics

Running example stable_fluid ...
usage: stable_fluid.py [-h] [-s]
stable_fluid.py: error: unrecognized arguments: example stable_fluid

So does the implicit mass-spring demo.

The reason is that the runpy.run_path method will pass the sys.argv which contains the arguments of the command: ti example ... to the example scripts and some of these arguments will not be recognized by the demos.

Here, I use a quick solution: use ArgumentParser.parse_known_args() instead of ArgumentParser.parse_args().

In the long run, I think we should support passing the arguments correctly from the ti CLI to examples since some arguments are important to the examples.

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[ailzhang]

LGTM!
Can you open an issue to track this and add a TODO in the comment?
Thanks!

[ailzhang]

Alternatively if we could only pass sys.argv[2:] to downstream( ofc add a comment saying that we skipped ti, example) that seems better?

[Hanke98]

Thanks!

Can you open an issue to track this and add a TODO in the comment?

Sure, I am working on that.

Alternatively if we could only pass sys.argv[2:] to downstream( ofc add a comment saying that we skipped ti, example) that seems better?

Good Idea! Let me write it to the issue. Thanks!

[ailzhang]

LGTM!
@Hanke98 would you mind helping sending a followup fix later as well ;) ? Thanks a lot!