Add size function to alpaka::Vec #6296

Job	Run time
clang-format	13s
linux_clang-14_cuda-11.2_debug_analysis	1m 26s
windows_cl-2022_debug_analysis	1m 42s
macos_xcode-14.2_debug_analysis	1m 41s
linux_gcc-12_debug_analysis	1m 30s
macos_xcode-14.2_release	1m 45s
macos_xcode-14.3.1_debug	1m 46s
windows_cl-2022_release	1m 43s
windows_cl-2022_debug	1m 36s
linux_gcc-11_debug	1m 6s
linux_gcc-12_release_c++20	1m 37s
linux_gcc-13_debug	1m 31s
linux_clang-13_release	1m 11s
linux_clang-14_release	1m 31s
linux_clang-15_debug	1m 32s
linux_clang-16_debug_ubsan	1m 26s
linux_clang-16_debug_tsan	1m 6s
	24m 22s

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