Fix DynamicCache reconstruction during ExecuTorch export (#47900)

* Preserve DynamicCache layer layouts in ExecuTorch export

* Clarify flattened cache test naming

* fixes

* clean ups

* fix test

* fix test

* Preserve cumulative length in ExecuTorch DynamicCache export

* Update ExecuTorch cache export signature expectations

* Revert ExecuTorch DynamicCache cumulative length preservation

* Use DDP cache data for ExecuTorch cache reconstruction

* simplify a bit

* comment

* fix type hint

* use exact type

* style

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Co-authored-by: Elad Segal <13485709+eladsegal@users.noreply.github.com>
Co-authored-by: Cyril Vallez <cyril.vallez@gmail.com>