Switching Github CI workflow to use nanobind. This should fix the err…

[Ra4ster]

Description

Moved CI workflow to nanobind from pybind.

Type of change

• Bug fix
• New feature
• Performance improvement
• Documentation
• Build/tooling
• Breaking change (existing C++ or Python API behavior changes)

Testing

Working to correct these:

• python build.py Release builds and DeepityTests passes
• python build.py Debug builds and DeepityTests passes (required for anything correctness-sensitive)
• pyright passes (if you touched bindings/, pydeepity/, examples/, or experiments/)

Platform(s) and compiler(s) tested:

Performance impact

None.

Checklist

• I've read CONTRIBUTING.md, including the portability rules and numerical gotchas relevant to this change.
• I've added a CHANGELOG.md entry under [Unreleased] (for any user-facing change).
• I've updated relevant docs (README, Doxygen comments) if behavior or the public API changed.
• New/changed code follows the project's .clang-format style.

Notes for reviewers

Also updated PyTorch's test_ffnn.py to use Sigmoid & ADAM. This is closer to the optimized MNIST test.