Update base for Update on "[ExecuTorch][to_backend] Enable passing Delegation Spec to to_backend"

Support Entire Graph Delegation Flow through EdgeProgramManager's to_backend.

### Motivation
A current usecase for backend lowering is through the `to_backend(backend_id, exported_program, compile_spec)` API which lowers the entire exported program to the specified backend_id. However, lowering via the EdgeProgramManager only allows for partitioner based lowering. the EdgeProgramManager is the main component which enables support for multiple methods, as a result backends which leverage the old `to_backend(backend_id, ...)` api can not export executorch models with multiple methods.

### Design
We override EdgeProgramManager to also allow Partitioner to be replaceable by DelegationSpec. DelegationSpec is essentially a wrapper around the backend_id and the compile_spec, so any where a partitioenr is specified to lower a graph, the delegation spec can also be used to do entier graph lowering.

### Intended Flow
```
del_spec = DelegationSpec("BackendWithCompilerDemo", [CompileSpec(...)])
encode_graph = torch.export.export(Encoder(), sample_inputs)
decode_graph = torch.export.export(Decoder(), sample_inputs)
edge_manager = to_edge({
    "encode": encode_graph,
    "decode": decode_graph,
})
lowered_edge_manager = edge_manager.to_backend(del_spec)
# or if you want to specify which methods to lower to with del_spec
lowered_edge_manager= edge_manager.to_backend({
    "encode": del_spec,
})
```

Differential Revision: [D69086565](https://our.internmc.facebook.com/intern/diff/D69086565/)

cc cccclai

[ghstack-poisoned]

Author: mcr229