Support named_data in flat_tensor (#10629)

Currently flat_tensor ndm only accounts for tensors in get_data, get_num_keys, get_key functions.
Add support to return named_data values as well.

TODO:consolidate tensors and named_data into one structure in the flatbuffer. This will simplify all the serialization and runtime code.

Differential Revision: [D73679683](https://our.internmc.facebook.com/intern/diff/D73679683/)

This PR was created by the merge bot to help merge the original PR into
the main branch.
ghstack PR number: https://github.com/pytorch/executorch/pull/10527 by
@lucylq
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ghstack PR base:
https://github.com/pytorch/executorch/tree/gh/lucylq/68/base
ghstack PR head:
https://github.com/pytorch/executorch/tree/gh/lucylq/68/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
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Co-authored-by: lucylq <lfq@meta.com>