[FEATURE REQUEST] `gemm` Python kernel implementation

[LoserCheems]

Problem statement

The BLAS level-3 gemm operation (general matrix-matrix multiply) currently has no pure Python reference implementation in kernel_course.python_ops. In the README BLAS table, the gemm row has no Python entry, even though gemm is a central building block for many higher-level algorithms.

Without a Python gemm kernel:

• there is no simple, framework-agnostic reference for $C = \alpha A B + \beta C$,
• backend implementations lack a canonical numerical baseline for correctness checks,
• educational materials cannot show a clear Python prototype for GEMM within this project.

Proposed solution

Add a Python reference implementation for gemm under kernel_course.python_ops, following the conventions of existing Python kernels.

Concretely:

• Introduce kernel_course/python_ops/gemm.py implementing $C = \alpha A B + \beta C$.
• Support 2D matrices A, B, and C with compatible shapes.
• Emphasize clarity and correctness over performance, as this is a reference implementation.

Alternatives considered

Using NumPy or PyTorch as the implicit reference would:

• tie semantics to a particular external library,
• diverge from the project’s pattern of explicit Python reference kernels,
• complicate reasoning about correctness independently of backend implementations.

Implementation details

• Add kernel_course/python_ops/gemm.py with a top-level gemm function.
• Accept scalars alpha, beta, matrices A, B, and C.
• Implement straightforward nested loops or a readable algorithm, including shape checks.
• Update kernel_course/python_ops/__init__.py to export gemm if appropriate.

Use case

The Python gemm kernel will:

• define the mathematical semantics of GEMM for all backends,
• provide a simple CPU-only reference for correctness and teaching,
• act as a baseline for testing and benchmarking backend implementations.

Related work

• Existing Python kernels: python_ops.copy, python_ops.swap.
• Standard BLAS gemm operations.

Additional context

This issue starts filling the gemm row in the README BLAS table with a Python reference implementation.