QNN EP can serialize graph to DLC (#24775)
### Description
This change adds support for serializing the QNN graph to the new Deep Learning Container (DLC) format. It is meant to supplement and perhaps eventually replace use of the QnnSaver backend, which emits C++ source
files when `qnn_saver_path` is set.
* Add support for serializing to .dlc via the QnnIr backend.
* Don't silently fallback to QnnCpu when QnnSaver was explicitly selected as the execution backend.
* Minor fixes.
### Motivation and Context
QNN model libraries, produced by compiling the C++ files that may be produced by QnnSaver have a number of drawbacks. Most importantly, they are not cross-platform and cannot be visualized via Netron or other tools. For these reasons, we anticipate that they may eventually be deprecated in favor of DLC files. These containers typically include a platform-agnostic representation of the graph QNN's internal representation.
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Co-authored-by: Jeff Kilpatrick <jkilpat@qti.qualcomm.com>
