Fix prepacked weight reference lifetime (#32040)
This pull request introduces a new method to the
`PrepackedWeightsForGraph` class to improve how prepacked weights are
managed, particularly when saving models. The main change is the
addition of a `DiscardAndReplaceWithReferenceIfSaving` method, which
either replaces a prepacked weight with a reference or removes it,
depending on whether the save mode is enabled. Corresponding updates are
made to usage sites and comprehensive unit tests are added to verify the
new behavior.
**Enhancements to prepacked weights management:**
* Added the `DiscardAndReplaceWithReferenceIfSaving` method to the
`PrepackedWeightsForGraph` class. This method replaces an existing entry
with a reference to another weight when in save mode, or removes it from
the container otherwise.
[[1]](diffhunk://#diff-beaf819b642665d791baf27f6931969a7ed44776f27939c16660038b2b76e760R111-R121)
[[2]](diffhunk://#diff-8f7cf3c97f7f9c93fd52a849ade7a4da4375b05f0debe357a8e52dd2090c8e2cR124-R129)
* Updated the call site in
`SessionState::PrepackConstantInitializedTensors` to use the new method,
ensuring correct handling of prepacked weights during model save
operations.
**Testing improvements:**
* Added new unit tests in `tensorutils_test.cc` to verify the behavior
of `DiscardAndReplaceWithReferenceIfSaving` in both save mode and
non-save mode, ensuring correct reference replacement and removal of
weights as appropriate.
