Fix Kokkos::Vector::push_back for default execution space

[masterleinad]

Fixes #5042. The main change is to allocate (and initialize) create modified_flags if it's not yet allocated in modify.
Without this, resize is copying the device data which has not been modified.

[dalg24]

Retest this please

[masterleinad]

@overfelt Can you confirm that this fixes your problem?

[overfelt]

Yes, I'll rerun my tests. Takes a while to build.

[overfelt]

Yes, this fixes my problem.

[dalg24]

OpenMPTarget-ROCm-4.5 failure seems unrelated

lld: error: /tmp/libkokkoscore-gfx906-ade9a8-gfx906-290beb.o: symbol _HASHW_/var/jenkins/workspace/Kokkos@2/core/unit_test/tools/TestEventCorrectness_cpp__omp_outlined___22_wrapper has undefined version 2/core/unit_test/tools/TestEventCorrectness_cpp__omp_outlined___22_wrapper
lld: error: /tmp/libkokkoscore-gfx906-ade9a8-gfx906-290beb.o: symbol _HASHW_/var/jenkins/workspace/Kokkos@2/core/unit_test/tools/TestEventCorrectness_cpp__omp_outlined___24_wrapper has undefined version 2/core/unit_test/tools/TestEventCorrectness_cpp__omp_outlined___24_wrapper
clang-13: error: amdgcn-link command failed with exit code 1 (use -v to see invocation)
make[2]: *** [core/unit_test/CMakeFiles/KokkosCore_UnitTest_KokkosP.dir/build.make:136: core/unit_test/KokkosCore_UnitTest_KokkosP] Error 1
make[1]: *** [CMakeFiles/Makefile2:2294: core/unit_test/CMakeFiles/KokkosCore_UnitTest_KokkosP.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....