Add constant folding for simple QDQ Node Units (#15138)
### Description
<!-- Describe your changes. -->
Currently we bail on constant folding if QDQ is enabled and we hit a DQ
node. However, if we have a simple DQ -> X -> Q node unit where the DQ
and X do not produce graph outputs, their output only has one consumer,
and X is deterministic, we can constant fold all three nodes.
Add support for this simple scenario primarily to constant fold a QDQ
model that has had initializers updated by layout transformation, which
results in patterns like `initializer -> DQ -> Transpose -> Q` or
`initializer- > DQ -> Unsqueeze -> Q -> DQ -> Transpose -> Q` if the
initializer is broadcast.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Improve end result of layout transformation on a QDQ model.
