Use NDM in backend init context (#9102)

Add NDM to backend init context, so backends have access to it at init time.

Add to backend init context instead of as an argument to backend.init, as it's easier to add/deprecate an arg there rather than in the init/execute interface.

TODO: add a merge/union method to named_data_map.h interface. If there is an external file (or multiple), and/or named_segments in the PTE, these should be merged into one.

Differential Revision: [D70279622](https://our.internmc.facebook.com/intern/diff/D70279622/)

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