[executorch][flat_tensor] Serialize flat tensor tests

[pytorchbot]

This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #7269
^ Please use this as the source of truth for the PR details, comments, and reviews
ghstack PR base: https://github.com/pytorch/executorch/tree/gh/lucylq/26/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/lucylq/26/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/main
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/lucylq/26/orig
@diff-train-skip-merge

[pytorch-bot]

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❌ 7 New Failures

As of commit 5d43ae2 with merge base 8494b90 ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9c9a9458801ff720d60e7a7c21eacdc0d49066724e23261fd9faa7daa5818d15 /exec failed with exit code 127
• pull / test-eval_llama-wikitext-linux / linux-job (gh)
  RuntimeError: Command docker exec -t fe063e9539958cf19cea39db0ab2509d939e2b4cc902de52823232db2118ed58 /exec failed with exit code 127
• pull / test-llama_runner_eager-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f418eecbb3d10f71e03df76264bb524b665a432c38118bcab4cc239d37de67ae /exec failed with exit code 127
• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ac4d089a2a2855ca4c5da82e503b6d31f0e1d0858a830c9ce4cc404448996d2f /exec failed with exit code 127
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 242fa4fd5a7a5b19022dd9a0168f7182f3571304fbce7092178b04da143031de /exec failed with exit code 127
• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d

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