Fix get_max_seq_len metadata method not found

[jackzhxng]

Summary

Fixes Llava not exporting with get_max_seq_len metadata method in the pte, which was happening because __post_init__ only works with @dataclass.

Testing

Exported a Llava pte and ran with the multimodal runner

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14192

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❌ 7 New Failures, 4 Cancelled Jobs, 37 Pending

As of commit dd8fc32 with merge base f294074 ():

NEW FAILURES - The following jobs have failed:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh)
  RuntimeError: Failed to install QNN SDK. Please check the logs above.
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t dedc6c02a61068261aad3b0dbf16f2c15547700cdbb5396906e3c97400128e96 /exec failed with exit code 127
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 3cbaa060f040c42bec6d51e40b8993b0dbe80efdbd8f25618a9a97e1a11f87ff /exec failed with exit code 127
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t d0aad7d85ba8d1ad7a1b54f9e39d3d84e548ccca21b945f16022bd6d20b171cf /exec failed with exit code 127
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4fa569f96285295500c0dba3418ae3acf9350d30d85386aa98f3a72ed78f47f4 /exec failed with exit code 23

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

• pull / test-binary-size-linux-gcc / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-moshi-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh)
  ##[error]The operation was canceled.

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