Dedup constants in emitter

[lucylq]

Summary

Fix bug where if we have two constants with different names but the same data, the second one is not saved and results in missing tensor. (This happened in quant flows).

Test plan

python -m unittest executorch.exir.emit.test.test_emit.TestEmit.test_constant_tagged_tensor_dedup 

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15139

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 6a66c8f with merge base c9339e2 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t 3122fbf187de44777bc701a129ddbe0a917ada91a4dbd7b08153fd9541a76d9c /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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