Android Refactor generate()

[kirklandsign]

Summary

We will no longer pass startPos, bos, eos args. User doesn't need to know that.

For generate_from_pos with image and text prompts, it will add input prompts first, and then invoke generate()

Test plan

CI

cc @cbilgin

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14183

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❌ 4 Cancelled Jobs, 3 Pending, 1 Unrelated Failure

As of commit a036eb9 with merge base 47acc87 ():

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

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-moshi-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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