[executorch][flat_tensor] DataMap implementation

Pull Request resolved: https://github.com/pytorch/executorch/pull/7900

DataMap implementation that
* Loads a flat_tensor file
* Makes tensor information available via the named_data_map.h interface.

TODO: in a later diff, update the ET runtime to hold onto the FreeableBuffers returned by the NDM. Then, the NDM will not persist the segment. T214294528

ghstack-source-id: 264916795

Differential Revision: [D67064580](https://our.internmc.facebook.com/intern/diff/D67064580/)

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