blas(axpby): execution space instance semantics not honored

[romintomasetti]

Using KokkosBlas::axpby, I could easily get a broken code if both following conditions are met:

1. use rank-0 views for the coefficients
2. pass an execution space instance on which these coefficients are being modified by some preceding kernel

The issue is clear if looking e.g. at the following line:

kokkos-kernels/blas/src/KokkosBlas1_axpby.hpp

Line 109 in b3a4bdf

InternalTypeA internal_a(Impl::getScalarValueFromVariableAtHost<AV, Impl::typeRank<AV>()>::getValue(a));

Adding an exec_space.fence() before fetching the value of the rank-0 view fixes it.

As a side node, I think you get potentially other issues, e.g. here

kokkos-kernels/blas/src/KokkosBlas1_axpby.hpp

Line 137 in b3a4bdf

Kokkos::deep_copy(managed_a, a);

where passing exec_space to Kokkos::deep_copy seems necessary.