[Doc] Add guidance for using ccache with pip install -e . in doc

[vadiklyutiy]

FIX #13697

ccache doesn't work when use with pip install -e .

Added hint to documentation:

When using ccache with pip install -e ., you should run CCACHE_NOHASHDIR="true" pip install --no-build-isolation -e .. This is because pip creates a new folder with a random name for each build, preventing ccache from recognizing that the same files are being built.

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[comaniac]

Thanks for the hint!

[ekagra-ranjan]

@vadiklyutiy @comaniac - when we use VLLM_USE_PRECOMPILED=1 pip install --editable ., it downloads the wheel file from the base commit in main branch. However, any local cpp/kernel changes will not be used as mentioned in the notes.

Do you think that if we use VLLM_USE_PRECOMPILED=1 CCACHE_NOHASHDIR="true" pip install --no-build-isolation -e . then the downloaded build files will be cached by ccache such that subsequent full build to build local cpp/kernel changes using CCACHE_NOHASHDIR="true" pip install --no-build-isolation -e . will be fast?

Basically, I am looking for a fast way to build a newly checkout of a vllm branch with some local cpp changes. Thanks!

[vadiklyutiy]

Do you think that if we use VLLM_USE_PRECOMPILED=1 CCACHE_NOHASHDIR="true" pip install --no-build-isolation -e . then the downloaded build files will be cached by ccache such that subsequent full build to build local cpp/kernel changes using CCACHE_NOHASHDIR="true" pip install --no-build-isolation -e . will be fast?

ccache don't see any downloaded files. It sees only what have come locally through gcc/nvcc/etc.