add example with run-time-parametrized matmul #6642

Job	Run time
ubuntu-22.04 llvm assert=ON rtti=ON	8m 58s
ubuntu-22.04 llvm assert=ON rtti=OFF	9m 7s
ubuntu-22.04 llvm assert=OFF rtti=ON	8m 57s
ubuntu-22.04 llvm assert=OFF rtti=OFF	8m 55s
ubuntu-22.04 gcc assert=ON rtti=ON	12m 45s
ubuntu-22.04 gcc assert=ON rtti=OFF	13m 10s
ubuntu-22.04 gcc assert=OFF rtti=ON	12m 35s
ubuntu-22.04 gcc assert=OFF rtti=OFF	13m 3s
ubuntu-24.04 llvm assert=ON rtti=ON	13m 56s
ubuntu-24.04 llvm assert=ON rtti=OFF	9m 0s
ubuntu-24.04 llvm assert=OFF rtti=ON	13m 40s
ubuntu-24.04 llvm assert=OFF rtti=OFF	9m 0s
ubuntu-24.04 gcc assert=ON rtti=ON	8m 39s
ubuntu-24.04 gcc assert=ON rtti=OFF	13m 26s
ubuntu-24.04 gcc assert=OFF rtti=ON	9m 0s
ubuntu-24.04 gcc assert=OFF rtti=OFF	12m 7s
ubuntu-20.04 gcc-11 assert=ON rtti=OFF	8m 50s
ubuntu-20.04 gcc-11 assert=OFF rtti=OFF	8m 53s
ubuntu-20.04 gcc-11 assert=ON rtti=ON	8m 49s
ubuntu-20.04 gcc-11 assert=OFF rtti=ON	8m 52s
windows-2019 msvc assert=ON rtti=ON	7m 58s
windows-2019 msvc assert=ON rtti=OFF	9m 9s
windows-2019 msvc assert=OFF rtti=ON	7m 28s
windows-2019 msvc assert=OFF rtti=OFF	8m 32s
	4h 4m 49s

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