[Executorch][Export][2/N] Add to_edge and to_backend stages

[abhinaykukkadapu]

Stack from ghstack (oldest at bottom):

• [Executorch][Export][3/N] Modularize export recipes #12938
• -> [Executorch][Export][2/N] Add to_edge and to_backend stages #12937
• [ExecuTorch][Export][1/N] Export API pipeline re-architecture, making it composable #12936

Address (6) in the rfc: #12660

1. Adds stage implementations for to_edge and to_backend
2. Adds unit tests for the two stages
3. Adds these two stages in the validation pipeline.

Fixes #12932

Differential Revision: D79120576

[pytorch-bot]

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❌ 7 New Failures, 1 Unrelated Failure

As of commit 36daa0d with merge base 45846c8 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (macos) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Check Labels / Check labels (gh)
  Process completed with exit code 1.
• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, cmake, linux.arm64.2xlarge, execut... / linux-job (gh)
  RuntimeError: Command docker exec -t 0eac1ac755fed2d5e56a72602eaef4118d6848d20a1f8833f7eb912942905909 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  export/tests/test_export_session.py::TestExportSessionCoreFlow::test_pipeline_validation_enforces_valid_transitions
• pull / unittest / macos / macos-job (gh)
  export/tests/test_export_session.py::TestExportSessionCoreFlow::test_pipeline_validation_enforces_valid_transitions
• pull / unittest-editable / linux / linux-job (gh)
  export/tests/test_export_session.py::TestExportSessionCoreFlow::test_pipeline_validation_enforces_valid_transitions
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_export_session.py::TestExportSessionCoreFlow::test_pipeline_validation_enforces_valid_transitions

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-phi-3-mini-runner-linux / linux-job (gh) (trunk failure)
  AttributeError: 'StaticCacheConfig' object has no attribute 'get'

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This pull request was exported from Phabricator. Differential Revision: D79120576