[executorch][emitter] Emit FQNs

Pull Request resolved: https://github.com/pytorch/executorch/pull/7192

Emit FQNs for external tensors. In the emitter, store external tensors as:
```
// list of unique tensors
external_constants_buffer: List[bytes]

// map of {constant_tag: {fqn: index into external_constant_buffer}}
// constant_tag: may want to save multiple external constant files; group them together via the tag.
// {fqn: index}; there may be multiple fqns pointing to the same data buffer. This is for deduplication.
external_constants_map: [Dict[str, Dict[str, int]]
```
ghstack-source-id: 257245125
@exported-using-ghexport

Differential Revision: [D66523226](https://our.internmc.facebook.com/intern/diff/D66523226/)

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