Enable exporting for inference when loading from buffer without behavior changes

[carzh]

Description

Added eval model buffer as optional field in Module so that you can export for inference using the eval model stored as a buffer.

Motivation and Context

• Resolves [Training] Cannot export model for inferencing from session created from buffers #21152
• Previous solution (PR Enable export for inference when eval model is loaded from buffer #21422) produced an eval model that was specific to the EP's used to train because of unavoidable runtime optimizations that changed the graph stored with the eval session.

[skottmckay]

Nice. Much cleaner.