Lazily load nvmath subpackages and bindings wrappers

[juenglin]

Summary

Importing any nvmath.bindings Cython submodule (for example CuPy's from nvmath.bindings.cycurand cimport ...) previously executed nvmath/__init__.py, which imported fft / linalg / sparse / tensor and every library wrapper, including optional *Mp and NVSHMEM modules. That mapped ~130 extra .so files (~350 MB RSS) into the process and slowed Python object construction process-wide (visible on cupy.empty_like and even on bytearray(256)).

This PR loads those subpackages and wrappers on first access (PEP 562). import nvmath.bindings.cycurand now maps cycurand and its own deps only. Public names, __all__, and "optional modules are None when unavailable" are unchanged. Native-library preload for host APIs still runs when nvmath.fft / linalg / sparse / tensor are actually imported.

Test plan

• pytest tests/nvmath_tests/test_bindings_init.py
• pytest tests/nvmath_tests/linalg/generic/matmul/test_qualifier.py
• pytest tests/nvmath_tests/linalg/test_layout.py
• pytest tests/nvmath_tests/ndbuffer/test_no_cupy.py
• A/B: import nvmath.bindings.cycurand — 130 → 7 extra .so, 347 MB → 8.6 MB RSS