FlatTensor alignment tests

[lucylq]

Summary:

1. Pass segment_alignment from ExecutorchBackendConfig to flat tensor serializer
2. Set segment_alignment=128 as default to match ExecutorchBackendConfig
3. Remove tensor_alignment from the config (do not have multiple tensors per segment)
4. Additional tests for varying segment alignment.

Differential Revision: D89422691

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit e334922 with merge base c61b2ed ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t e783adf6c06b5dd63b0378dfe0a5b9417eca3dd3a61548c16c6e7316097d0585 /exec failed with exit code 1

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[meta-codesync]

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[larryliu0820]

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