Update base for Update on "[executorch][flat_tensor] implement load into and dont hold onto the segment"

1. Implement load_into in FlatTensorDataMap
2. Do not persist 'data_ro' in the FlatTensorDataMap. From `get_data`, return the FreeableBuffer given by the data loader.

TODO: add test for load_into.

Differential Revision: [D69148652](https://our.internmc.facebook.com/intern/diff/D69148652/)

[ghstack-poisoned]

Author: lucylq