[FEATURE REQUEST] `gemm` test coverage and fixtures

[LoserCheems]

Problem statement

The gemm kernel currently has no dedicated tests under tests/. There is no test_gemm.py to validate that all backends correctly implement $C = \alpha A B + \beta C$.

Without gemm tests:

• regressions or subtle numerical differences across backends may go unnoticed,
• CI cannot guarantee that GEMM implementations continue to match the Python reference,
• the README BLAS table cannot mark the Test column for gemm as complete.

Proposed solution

Add a dedicated pytest module for gemm that validates all available backends against the Python reference implementation.

Concretely:

• Introduce tests/test_gemm.py alongside other kernel tests.
• For each backend (Python, PyTorch, Triton, CuTe when available), compare outputs with the Python gemm reference for a variety of matrix sizes, shapes, and scalar values.
• Cover edge cases (e.g. small matrices, rectangular matrices, zero scalars, and different aspect ratios).

Alternatives considered

Manual or informal testing is not sufficient because it:

• is easy to skip during development,
• does not integrate with CI,
• lacks systematic regression coverage for this critical kernel.

Implementation details

• Add tests/test_gemm.py using pytest conventions.
• Reuse helpers for random input generation and output comparison from other tests where possible.
• Parameterize tests over matrix dimensions, dtypes, and scalar values.
• Gracefully skip backend-specific tests when a backend is not installed or not yet implemented.

Use case

The gemm tests will:

• enforce correctness for GEMM across all backends,
• provide confidence to contributors implementing and optimizing GEMM kernels,
• support the project’s focus on numerical equivalence as performance improvements are made.

Related work

• Existing tests: tests/test_copy.py, tests/test_swap.py.
• BLAS gemm test suites in other numerical libraries.

Additional context

This issue is part of fully populating the gemm row in the README BLAS table, particularly the Test column.