[v2.6.x] Upgrade to CUDA 12.9

[regro-cf-autotick-bot]

This PR has been triggered in an effort to update cuda129.

Notes and instructions for merging this PR:

1. Please merge the PR only after the tests have passed.
2. Feel free to push to the bot's branch to update this PR if needed.

Please note that if you close this PR we presume that the feedstock has been rebuilt, so if you are going to perform the rebuild yourself don't close this PR until the your rebuild has been merged.

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Here are some more details about this specific migrator:

CUDA 12.8 added support for architectures sm_100, sm_101 and sm_120,
while CUDA 12.9 further added sm_103 and sm_121. To build for these,
maintainers will need to modify their existing list of specified architectures
(e.g. CMAKE_CUDA_ARCHITECTURES, TORCH_CUDA_ARCH_LIST, etc.)
for their package. A good balance between broad support and storage
footprint (resp. compilation time) is to add sm_100 and sm_120.

Since CUDA 12.8, the conda-forge nvcc package now sets CUDAARCHS and
TORCH_CUDA_ARCH_LIST in its activation script to a string containing all
of the supported real architectures plus the virtual architecture of the
latest. Recipes for packages who use these variables to control their build
but do not want to build for all supported architectures will need to override
these variables in their build script.

ref: https://docs.nvidia.com/cuda/cuda-toolkit-release-notes/index.html#new-features

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If this PR was opened in error or needs to be updated please add the bot-rerun label to this PR. The bot will close this PR and schedule another one. If you do not have permissions to add this label, you can use the phrase @conda-forge-admin, please rerun bot in a PR comment to have the conda-forge-admin add it for you.

_{This PR was created by the regro-cf-autotick-bot. The regro-cf-autotick-bot is a service to automatically track the dependency graph, migrate packages, and propose package version updates for conda-forge. Feel free to drop us a line if there are any issues! This PR was generated by https://github.com/regro/cf-scripts/actions/runs/16527768707 - please use this URL for debugging.}

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The recipe is not parsable by parser conda-souschef (grayskull). This parser is not currently used by conda-forge, but may be in the future. We are collecting information to see which recipes are compatible with grayskull.
• ℹ️ The recipe is not parsable by parser conda-recipe-manager. The recipe can only be automatically migrated to the new v1 format if it is parseable by conda-recipe-manager.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/16528109065. Examine the logs at this URL for more detail.}

[iamthebot]

Okay, so looks like the issue causing OSX builds to fail was addressed upstream in 2.7+ releases here.

Created a patch and testing it. Will also test CUDA builds (I have access to a big GPU box to build on)

[iamthebot]

Okay so #408 does fix the OSX build issue; however, seems a small number (4) tests are failing. @h-vetinari any chance we have logs from the linux builds?