PR builds use oldest python instead of latest python for generate_binary_build_matrix.py

[jithunnair-amd]

As per generate_binary_build_matrix.py:

Will output a condensed version of the matrix if on a pull request that only
includes the **latest** version of python we support built on four different
architectures:
    * CPU
    * Latest CUDA
    * Latest ROCM
    * Latest XPU

However, the code actually selects the first python version in the list, which is the oldest:

test-infra/tools/scripts/generate_binary_build_matrix.py

Line 22 in 81ebba3

PYTHON_ARCHES_DICT = {

test-infra/tools/scripts/generate_binary_build_matrix.py

Line 466 in 81ebba3

python_versions = [python_versions[0]]

This runs py3.9 instead of py3.12, which doesn't seem to be what is intended according to the header eg. https://github.com/pytorch/test-infra/actions/runs/12191615067?pr=6016

[huydhn]

I remembered asking @atalman about this a while ago, I think we choose the oldest one there because the latest one might be experimental one like 3.13+. So, maybe we just need to update the doc to reflect that. Also, testing all version can be done by adding ciflow/binaries or ciflow/binaries/all into the PR.

[jithunnair-amd]

I remembered asking @atalman about this a while ago, I think we choose the oldest one there because the latest one might be experimental one like 3.13+. So, maybe we just need to update the doc to reflect that. Also, testing all version can be done by adding ciflow/binaries or ciflow/binaries/all into the PR.

Yes, that makes sense to me. Also, not all the builds would support the latest python version at any point of time eg. python13 is currently not supported by ROCm and other builds.