Run pytest on NVIDIA self-hosted runner

[voltjia]

Summary

This PR routes the pytest GitHub Actions workflow to the new NVIDIA self-hosted runner for trusted contexts. The runner is registered as nvidia-0 with labels nvidia, gpu, cuda, and ninetoothed.

Changes:

• Move pytest from ubuntu-latest to [self-hosted, linux, x64, nvidia, ninetoothed].
• Skip self-hosted pytest for fork PRs to avoid running untrusted code on the GPU host.
• Use the runner image’s python3 environment instead of actions/setup-python, so CI can reuse the NVIDIA/PyTorch container environment.

Validation:

• Confirmed nvidia-0 is online in GitHub Actions.
• Confirmed the remote runner container is running with --restart unless-stopped.
• Verified pytest passes on the self-hosted runner: https://github.com/InfiniTensor/ninetoothed/actions/runs/26500802071

Testing

pytest output:

============================= test session starts ==============================
platform linux -- Python 3.12.3, pytest-8.1.1, pluggy-1.6.0
rootdir: /home/runner/actions-runner/_work/ninetoothed/ninetoothed
configfile: pyproject.toml
plugins: cov-7.1.0, typeguard-4.4.4, anyio-4.12.0, hypothesis-6.130.8, shard-0.1.2, flakefinder-1.1.0, xdoctest-1.0.2, xdist-3.8.0, rerunfailures-16.1
collected 216 items
Running 216 items in this shard

tests/test_add.py .                                                      [  0%]
tests/test_addmm.py ..                                                   [  1%]
tests/test_aot.py ............                                           [  6%]
tests/test_aot_auto_tuning.py ....                                       [  8%]
tests/test_attention.py ........                                         [ 12%]
tests/test_auto_tuner.py ....                                            [ 14%]
tests/test_clone.py ....                                                 [ 16%]
tests/test_conv2d.py ....                                                [ 18%]
tests/test_data_ptr.py .                                                 [ 18%]
tests/test_debugging.py .                                                [ 18%]
tests/test_dropout.py .                                                  [ 19%]
tests/test_eval.py ........                                              [ 23%]
tests/test_expand.py .                                                   [ 23%]
tests/test_generation.py ............................................... [ 45%]
.............................                                            [ 58%]
tests/test_getitem.py ..........                                         [ 63%]
tests/test_ipynb.py .                                                    [ 63%]
tests/test_jagged.py ................                                    [ 71%]
tests/test_matmul.py ..                                                  [ 72%]
tests/test_max_pool2d.py ..                                              [ 73%]
tests/test_naming.py .......                                             [ 76%]
tests/test_pad.py ................................................       [ 98%]
tests/test_pow.py .                                                      [ 99%]
tests/test_softmax.py .                                                  [ 99%]
tests/test_unsqueeze.py .                                                [100%]

- generated xml file: /home/runner/actions-runner/_work/ninetoothed/ninetoothed/junit/test-results.xml -
================================ tests coverage ================================
_______________ coverage: platform linux, python 3.12.3-final-0 ________________

Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml
======================= 216 passed in 1968.77s (0:32:48) =======================