refactor preprocess to use EagerModelBase

[lucylq]

Stack from ghstack (oldest at bottom):

• -> refactor preprocess to use EagerModelBase #6536

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6536

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❌ 2 New Failures

As of commit dbe4df4 with merge base 85d3ff6 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  ##[error]Process completed with exit code 1.
• pull / test-preprocess-linux / linux-job (gh)
  test_preprocess

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