Add FP8 placeholder support to ExecuTorch serialization

[YufengShi-dudu]

Change-Id: Ibb7ef4167ab96426133fce64e34366c365cd12ad

cc @JacobSzwejbka @angelayi

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19043

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• Rolling out OSDC (ARC) runners on pull & trunk workflows in PyTorch main

❌ 2 New Failures, 2 Cancelled Jobs, 3 Unrelated Failures

As of commit 954e64d with merge base 32a6cec ():

NEW FAILURES - The following jobs have failed:

• trunk / test-models-macos-coreml (vit) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-cpu (resnet50, xnnpack-quantization-delegation) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

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 / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[YufengShi-dudu]

Hey @digantdesai, could you take a look at this RP? Thank you