[DML EP] Remove unconnected nodes from the graph (#14155)
### Description
Remove unconnected nodes from the DML EP graph.
### Motivation and Context
Some operators like `EmbedLayerNorm` have many outputs, and some of the
outputs are non-optional. But in practice, they act like optional
outputs because they can have a value of 0, which means that the rest of
the model doesn't need to depend on those. The problem with that is that
DML will implicitly remove those output from the graph, but the nodes
that feed into that output will stay and become unconnected from the
rest of the graph, which is illegal in DML. Removing unconnected nodes
as a last pass will make sure that those nodes are getting removed and
will simplify the logic of individual operators by not having to account
for these special cases.
