Dedup delegate blobs in emitter

[pytorchbot]

Summary:
Previously we deduplicated entire 'BackendDelegate' blobs using the preprocessed blob. If two BackendDelegate fields have different id or compile specs, it would be disregarded.

This diff:

1. Only deduplicates the preprocessed blob. BackendDelegate retains its own compile specs, etc.
2. Removes the per-method 'delegate_cache', as we have a program-wide delegate cache.
3. Adds a test to confirm we have one delegate segment but two BackendDelegate references pointing at it.

Differential Revision: D83162107

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14658

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 2 Cancelled Jobs

As of commit 7473e17 with merge base e0dda90 ():

NEW FAILURE - The following job has failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  test_split_group_convolution__2d_1_group_3

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
  ##[error]The operation was canceled.
• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[GregoryComer]

Rebasing to re-test after mac job fix.