changes to matmul codebase: reducing line-count

[oleksandr-pavlyk]

Used if constexpr to make m_groups=1 specializations of functors unnecessary.

Also used const references for vectors in some places.

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 91.065%. remained the same
when pulling e3f4160 on gemm-cleanup
into 6089e81 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.15.1dev3=py310h15de555_69 ran successfully.
Passed: 907
Failed: 2
Skipped: 86

[ndgrigorian]

LGTM!