Add flag to disable constant buffer

[lucylq]

Summary

Add flag to disable constant buffer.

Test plan

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19522

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❗ 1 Active SEVs

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• Run pull request jobs on OSDC runners in shadow mode

❌ 3 New Failures, 3 Unrelated Failures, 1 Unclassified Failure

As of commit f3a7340 with merge base bd5752a ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / macos / macos-job (gh)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest / windows / windows-job (gh)
  [ FAILED ] ProgramTest.GetConstantBufferDataRejectsOversizedRequest
• pull / unittest-editable / windows / windows-job (gh)
  [ FAILED ] ProgramTest.GetConstantBufferDataRejectsOversizedRequest

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Check Labels / Check labels (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: GraphQL query

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

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

• pull / unittest / linux / linux-job (gh) (trunk failure)
  exir/tests/test_joint_graph.py::TestJointGraph::test_joint_graph
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  exir/tests/test_memory_planning.py::TestMisc::test_multiple_pools_1

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