Use a temp directory for CMake build

[fheinecke]

Description

CMake currently always uses ./build/cmake as a build directory when NVTE_CMAKE_BUILD_DIR is not set. This causes a couple of issues:

• Builds can collide with each other
• Generated files can become stale and require cleaning prior to every build
• Build files are not cleaned up upon success nor failure

This creates a temporary directory and cleans it up afterwards. Note: this makes it harder to inspect the build directory, because it will now be cleaned up after every run. For this use case I would recommend setting `NVTE_CMAKE_BUILD_DIR="${PWD}/build/cmake" to retain the current behavior. As a result of moving to a temp directory, object files will now not be reused between builds as well.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• CMake extension builds are now isolated

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[fheinecke]

/te-ci

[fheinecke]

The failing job is a scheduling timeout for a GPU-specific runner that was having some issues next week. Going to assume that it's unrelated.

[fheinecke]

/te-ci

[greptile-apps]

Greptile Summary

The PR isolates non-editable CMake extension builds in automatically cleaned temporary directories while retaining persistent incremental builds for editable installs or when NVTE_CMAKE_BUILD_DIR is configured.

• Creates unique CMake build directories beneath setuptools’ build-temp directory.
• Cleans temporary CMake state after successful or failed builds.
• Preserves build/cmake for in-place builds and explicit build-directory overrides.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
build_tools/build_ext.py	Selects persistent or temporary CMake build directories based on environment and editable-build state, with scoped automatic cleanup.

_{Reviews (2): Last reviewed commit: "Add fallback to exisiting build dir beha..." | Re-trigger Greptile}

[ptrendx]

Hmm, this has a potential of being a little disruptive (especially considering that the time needed to build TE is quite long). Could there maybe be some middle ground where e.g. the -e builds (develop and in-place) could use the in-place build directory and the regular installation would use the temp instead?

[fheinecke]

Good catch - added

[fheinecke]

/te-ci