[Doc] [cc] Doc the workflow for exporting Taichi kernels to C99 source

[archibate]

Related issue = #920 #1644

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[codecov]

Codecov Report

Merging #1756 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1756      +/-   ##
==========================================
- Coverage   43.64%   43.63%   -0.01%     
==========================================
  Files          44       44              
  Lines        6051     5940     -111     
  Branches     1083     1071      -12     
==========================================
- Hits         2641     2592      -49     
+ Misses       3255     3195      -60     
+ Partials      155      153       -2     

Impacted Files	Coverage Δ
python/taichi/cc_compose.py	0.00% <0.00%> (ø)
python/taichi/main.py	23.91% <0.00%> (+0.50%)	⬆️
python/taichi/lang/kernel.py	70.22% <0.00%> (-0.96%)	⬇️
python/taichi/misc/util.py	17.17% <0.00%> (-0.58%)	⬇️
python/taichi/code_format.py	12.00% <0.00%> (-0.38%)	⬇️
python/taichi/lang/transformer.py	81.61% <0.00%> (-0.28%)	⬇️
python/taichi/lang/common_ops.py	69.04% <0.00%> (-0.22%)	⬇️
python/taichi/lang/impl.py	66.76% <0.00%> (-0.09%)	⬇️
python/taichi/diagnose.py	0.00% <0.00%> (ø)
python/taichi/misc/gui.py	0.00% <0.00%> (ø)
... and 8 more

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[k-ye]

Will this be most helpful to @samuela (#1642)? If so, better inviting him to help take a look?

[samuela]

Thanks for putting this together @archibate ! My one big-picture comment is that it'd be nice to also include a note on which Taichi features are and are not supported by the C backend at the moment. Eg, my understanding is that the AD stack does not yet work with the C backend, but that most other things do. Another question is whether it's possible for the C backend to output code CUDA-compatible code.

[rexwangcc]

I left a few comments around wording, thank you for updating the documentation for CC backend, so excited!

[yuanming-hu]

Awesome! I can't wait to try it. Please fix the minor wording issues. Thank you so much!

[yuanming-hu]

Great! LGTM now. Thank you very much!