Update on "Backend data separation test"

Add backend data separation test with demo ExecutorBackend.

Note: ExecutorBackend is a wrapper around a portable PTE file. It doesn't support delegated program-data separation in the sense of tagging tensors as external and placing them into the named data store as such. Rather, this test:

1. Creates a linear PTE file with data removed, using the portable flow.
2. Packages (1) into preprocessed blob as a delegate, using ExecutorBackend. Note: this discards the PTD portion.
3. Re-create the PTD portion via export_program (portable flow again).
4. Runs the delegated ExecutorBackend linear with portable linear.ptd file.

Caveat: this means that LinearModule definition in export_program and export_delegated_program must stay in sync, as we get the PTE artifact from export_delegated_program (wrapped in ExecutorBackend), and the PTD artifact from export_program.

Differential Revision: [D73679733](https://our.internmc.facebook.com/intern/diff/D73679733/)

[ghstack-poisoned]

Author: lucylq

test/models/export_delegated_program.py

@@ -24,6 +24,9 @@
 from executorch.exir.backend.test.backend_with_compiler_demo import (
     BackendWithCompilerDemo,
 )
+from executorch.exir.backend.test.demos.rpc.executor_backend_preprocess import (
+    ExecutorBackend,  # noqa: F401
+)
 from executorch.exir.passes.external_constants_pass import (
     delegate_external_constants_pass,
 )