Update base for Update on "[executorch][flat_tensor] DataMap implementation"

DataMap implementation that
* Loads a flat_tensor file
* Populates a map with {fqn: tensor} and {fqn: TensorLayout}.
* Makes tensor information available via the named_data_map.h interface.

For now, DataMap doesn't store the DataLoader.
- If/when tensors are in their own segments, DataMap should also store a DataLoader.

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

[ghstack-poisoned]

Author: lucylq