Update on "Add preprocess to ci"

1. Run `test_preprocess.py` in ci.
2. Refactor `test_preprocess.py` by creating the model artifacts (eager, exported, et) once and re-use them on different inputs (previously, create model artifacts for each test).

Test Plan:
```
pytest -c /dev/null -v -n auto examples/models/llama3_2_vision/preprocess/
```

[ghstack-poisoned]

Author: lucylq

.github/workflows/pull.yml

@@ -231,34 +231,6 @@ jobs:
         # run e2e (export, tokenizer and runner)
         PYTHON_EXECUTABLE=python bash .ci/scripts/test_llava.sh
 
-  test-preprocess-linux:
-    name: test-preprocess-linux
-    uses: pytorch/test-infra/.github/workflows/linux_job.yml@main
-    strategy:
-      fail-fast: false
-    with:
-      runner: linux.24xlarge
-      docker-image: executorch-ubuntu-22.04-clang12
-      submodules: 'true'
-      ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
-      timeout: 90
-      script: |
-        # The generic Linux job chooses to use base env, not the one setup by the image
-        CONDA_ENV=$(conda env list --json | jq -r ".envs | .[-1]")
-        conda activate "${CONDA_ENV}"
-
-        PYTHON_EXECUTABLE=python bash .ci/scripts/setup-linux.sh "cmake"
-
-        # install pybind
-        bash install_requirements.sh --pybind xnnpack
-
-        # install preprocess requirements
-        bash examples/models/llama3_2_vision/install_requirements.sh
-
-        # run python unittest
-        pytest -c /dev/null -v -n auto examples/models/llama3_2_vision/preprocess/
-
-
   test-quantized-aot-lib-linux:
     name: test-quantized-aot-lib-linux
     uses: pytorch/test-infra/.github/workflows/linux_job.yml@main