add readme for gemma3 cuda

[Gasoonjia]

Summary: creates a readme for exporting and running gemma3 on cuda backend

Differential Revision: D85220287

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15349

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 307a295 with merge base 81a3acc ():

NEW FAILURE - The following job has failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t bf4b84a861e0d7c2e3764e79068b5f4753c3cc1b77207e18de5d0f9714a1bf3f /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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

• pull / unittest / linux / linux-job (gh) (trunk failure)

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[meta-codesync]

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[github-actions]

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