[executorch][serialization] Serialize PTD files.

Pull Request resolved: https://github.com/pytorch/executorch/pull/7270

Introduce top-level serialization file that calls:
- serialize_pte_binary for PTE file
- FlatTensor.serialize_tensors for PTD files.


ghstack-source-id: 262004271
@exported-using-ghexport

Differential Revision: [D66523267](https://our.internmc.facebook.com/intern/diff/D66523267/)

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Co-authored-by: lucylq <lfq@meta.com>