[CLI] Add example command to CLI

[rexwangcc]

Related issue = #775

Implementation plan and thoughts

• First iteration to make something like ti example mpm88 work, with dead-simple invalid argument handling (ti example fake). Leveraging the existing condition branches for simplicity.
• Update dev_install docs as I setup development environment. (Please let me know if this makes the PR too convoluted, I could remove and cherry-pick it from a separate [doc] PR)
• [ ] Instead of using the long-winded if-elif-else, move the example subcommand to a more Pythonic argparse based CLI. This will need insights from the core devs since it requires larger refactor, and might be ideal to implement in a separate PR. out of scope
• Also could be moved to a spearate task, ideally the examples files could be registered someway, instead of being called by os.system(), which is error-prone. I ended up using runpy with absolute paths.
• [ ] Add tests for CLI for example command. out of scope
• [ ] The order of returned set of available example names when it errors out is not guaranteed, this is more of an UX question than a technical problem. I'll leave it as it is for now. out of scope
• ti example is now runnable from anywhere on the system, as long as the script is registered.

Yak shaving

• Fixed a missing shutil import for convert subcommand.

Testing commands

ti example 
ti example fake-example
ti example bitmasked
ti example cornell_box
ti example euler
ti example fractal
ti example game_of_life
ti example image_io
ti example keyboard
ti example laplace
ti example lists
ti example mgpcg
ti example mgpcg_advanced
ti example minimal
ti example minimization_trivial
ti example mlp
ti example mnist
ti example mpm128
ti example mpm88
ti example mpm99
ti example mpm_lagrangian_forces
ti example nbody_oscillator
ti example particle_renderer
ti example pbf2d
ti example pytorch_tensor_ad
ti example pytorch_tensor_io
ti example quadtree
ti example regression
ti example renderer_utils
ti example sdf2d
ti example sdf_renderer
ti example simple_autodiff
ti example stable_fluid
ti example taichi_logo
ti example taichi_sparse
ti example waterwave

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[archibate]

Also could be moved to a spearate task, ideally the examples files could be registered someway, instead of being called by os.system(), which is error-prone.

To run a file in python without os.system(), you may want to try this:

exec(f'import examples.{name}')

(see Suite.__init__ in benchmark/run.py)

[rexwangcc]

@archibate Thanks for your insights! Apparently exec worked better for os.system() from the compatibility perspective, inspired by which I ended up using importlib for the sake of avoid exec/eval as much as I could.

Per:

Instead of using the long-winded if-elif-else, move the example subcommand to a more Pythonic argparse based CLI. This will need insights from the core devs since it requires larger refactor, and might be ideal to implement in a separate PR.

I'd like to get your insights, especially on whether we shall do a refactor in this PR or a seprate one. Right now I don't see any tests for the CLI, which makes the development around it a bit tricky. Ideally if we could switch to something like:

commands = parser.add_subparsers(help='Available Taichi commands')
examples = commands.add_parser('examples', help='Interact with Taichi examples')
examples.add_argument(****)

for most of the commands/flags/arguments, it will be much eaiser to unit-test them and it's less error-prone.

I left example command in the if-elif-else blocks since moving it to above style requires refactoring on other commands too, I figured it's preferred to have them refactored altogether in a separate PR.

[rexwangcc]

Hi folks! This PR is ready for review, please take a look when you get a chance, thanks 😊

[yuanming-hu]

Thank you! This is very helpful. LGTM in general except for a few minor issues.

[yuanming-hu]

Thank you so much! Everything looks good to me now. After adding this line https://github.com/taichi-dev/taichi/pull/1010/files#diff-2eeaed663bd0d25b7e608891384b7298R19
Your CLI works when Taichi is installed as a Python package. I'll merge after CI passes!