Missing dependency for cmake build in documentation.

[bluejack]

📚 The doc issue

A dependency for the cmake build:

pip install zstd

Build will fail in a confusing way without this dependency prepared.

Suggest a potential alternative/fix

Add to documentation, probably here:

https://pytorch.org/executorch/stable/runtime-build-and-cross-compilation.html#one-time-setup-to-prepare-for-cmake-build

cc @mergennachin @byjlw

[byjlw]

Thanks for reporting it.

[swolchok]

Weird, zstd is listed as a build-system dep in pyproject.toml. I wonder what's going wrong...

[swolchok]

@bluejack can you provide the error you get if you don't have zstd installed?

[bluejack]

Well, gosh! Now I cannot repro this problem with the release build or with the repo head. I hit it with build attempts on both branches, so whatever I was doing wrong I was repeating my mistake quite effectively.

I apologize for the confusion!