Add size function to alpaka::Vec #6298

Job	Run time
clang-format	13s
linux_clang-14_cuda-11.2_debug_analysis	19m 2s
windows_cl-2022_debug_analysis	2h 33m 7s
macos_xcode-14.2_debug_analysis	26m 22s
linux_gcc-12_debug_analysis	21m 11s
macos_xcode-14.2_release	27m 23s
macos_xcode-14.3.1_debug	29m 35s
windows_cl-2022_release	2h 31m 29s
windows_cl-2022_debug	2h 27m 2s
linux_gcc-11_debug	19m 13s
linux_gcc-12_release_c++20	21m 7s
linux_gcc-13_debug	17m 55s
linux_clang-13_release	18m 26s
linux_clang-14_release	20m 49s
linux_clang-15_debug	15m 0s
linux_clang-16_debug_ubsan	17m 47s
linux_clang-16_debug_tsan	16m 10s
	12h 1m 51s

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