onnxruntime

Fix CPU constant folding not reverting the node to its previous EP (#17399)

A recent change was made in
https://github.com/microsoft/onnxruntime/commit/5a83a67f32da784b7dd4029bb23e7cc05363b748
to make `ep_type` a reference instead of having it be a copy, presumably
to avoid assigning strings (so `auto& ep_type =
node->GetExecutionProviderType()` instead of `auto ep_type =
node->GetExecutionProviderType()`). The problem with this change is that
calling `node->SetExecutionProviderType(kCpuExecutionProvider)` will
change the value of the reference itself, which means that it's
impossible to revert the node to its previous EP.

This change fixes this bug and adds an optimization over the previous
approach by only assigning a string when we know that we are dealing
with a non-CPU node.