[fix] fix import error cased by _load_cuda_libs when running pytest

[Darryl233]

Description

When using pytest to test CI, the working directory is the TransformerEngine-FL project directory, and te_path points to the local source tree TransformerEngine-FL/. There are no .so files in the source tree (they are in build/ or site-packages), which causes it to return False.
Fixes # (issue)

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:

• Add a TE_LIB_PATH environment variable to manually override TE_PATH during pytest execution.

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

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ Darryl233
❌ liyuzhuo

---

liyuzhuo seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[lxd-cumt]

In this PR, only the te_path in vendor/cuda was updated.
Don't the te_path entries under vendor/ for other hardware vendors (such as hygon) also need to be updated?