Allow EpContext models with input/output models completely in buffers (#24463)
### Description
Re-enables (and fixes) generation of compiled EpContext models with
**both** input and output models stored in buffers.
### Motivation and Context
Previous PR #24176 inadvertently added a check that disabled storing
both input and output models in buffers. However, we need this
functionality. This was actually a fortunate scenario, as it led to the
discovery of a bug.