Add Python build and GPU test CI

[scal444]

Builds nvMolKit from source and runs the non-long Python pytest suite on the colossus GPU runner.

[greptile-apps]

Greptile Summary

The PR adds a GPU-backed GitHub Actions workflow that builds nvMolKit from source and runs its non-long Python test suite.

• Detects the available GPUs and configures corresponding CUDA architectures.
• Installs the Conda, native, Python, and test dependencies.
• Verifies PyTorch GPU access before building and testing the package.
• Pins the checkout action to an immutable commit.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
.github/workflows/python-build-test.yml	Adds the Colossus GPU build-and-test workflow; the previously reported actionable checkout concern is fixed and no blocking failure remains.

_{Reviews (4): Last reviewed commit: "Modernize Python build and test CI" | Re-trigger Greptile}

[scal444]

Wraps up #158