Add matmul operator

[voltjia]

pytest output:

============================= test session starts ==============================
platform linux -- Python 3.12.3, pytest-8.3.5, pluggy-1.5.0
benchmark: 5.1.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /home/voltjia/ntops
configfile: pyproject.toml
plugins: jaxtyping-0.3.2, hypothesis-6.141.0, anyio-4.9.0, cov-6.0.0, benchmark-5.1.0
collected 8 items

tests/test_matmul.py ........                                            [100%]

============================== 8 passed in 2.63s ===============================