[Misc][Build] Lazy load cv2 in nemotron_parse.py

[kiersten-stokes]

Purpose

Lazy loads the cv2 module to avoid import errors in certain envs when not needed

Error trace

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.12/multiprocessing/spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "/usr/lib/python3.12/multiprocessing/spawn.py", line 131, in _main
    prepare(preparation_data)
  File "/usr/lib/python3.12/multiprocessing/spawn.py", line 246, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "/usr/lib/python3.12/multiprocessing/spawn.py", line 297, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen runpy>", line 280, in run_path
  File "<frozen runpy>", line 92, in _run_module_code
  File "<frozen runpy>", line 86, in _run_code
  File "/opt/venv/bin/vllm", line 5, in <module>
    from vllm.scripts import main
  File "/opt/venv/lib/python3.12/site-packages/vllm/__init__.py", line 3, in <module>
    from vllm.engine.arg_utils import AsyncEngineArgs, EngineArgs
  File "/opt/venv/lib/python3.12/site-packages/vllm/engine/__init__.py", line 1, in <module>
    from vllm.engine.async_llm_engine import AsyncLLMEngine
  File "/opt/venv/lib/python3.12/site-packages/vllm/engine/async_llm_engine.py", line 11, in <module>
    from vllm.executor.executor_base import ExecutorBase
  File "/opt/venv/lib/python3.12/site-packages/vllm/executor/__init__.py", line 1, in <module>
    from vllm.executor.executor_base import ExecutorBase, ExecutorAsyncBase
  File "/opt/venv/lib/python3.12/site-packages/vllm/executor/executor_base.py", line 7, in <module>
    from vllm.lora.request import LoRARequest
  File "/opt/venv/lib/python3.12/site-packages/vllm/lora/__init__.py", line 1, in <module>
    from vllm.lora.layers import (
  File "/opt/venv/lib/python3.12/site-packages/vllm/lora/layers.py", line 14, in <module>
    from vllm.model_executor.layers.linear import (
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/__init__.py", line 1, in <module>
    from vllm.model_executor.models import ModelRegistry
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/models/__init__.py", line 9, in <module>
    _MODELS = {
              ^
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/models/__init__.py", line 127, in <dictcomp>
    model: _get_model_architecture(model) for model in _MODELS
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/models/__init__.py", line 127, in <dictcomp>
    model: _get_model_architecture(model) for model in _MODELS
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/models/__init__.py", line 35, in _get_model_architecture
    mod = importlib.import_module(
          ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 995, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/opt/venv/lib/python3.12/site-packages/vllm/model_executor/models/nemotron_parse.py", line 14, in <module>
    import cv2
  File "/opt/venv/lib/python3.12/site-packages/cv2/__init__.py", line 181, in <module>
    bootstrap()
  File "/opt/venv/lib/python3.12/site-packages/cv2/__init__.py", line 153, in bootstrap
    native_module = importlib.import_module("cv2")
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/cv2/typing/__init__.py", line 62, in <module>
    import cv2.mat_wrapper
ImportError: libxcb.so.1: cannot open shared object file: No such file or directory

Test Plan

Build locally in problem env and test for above error

Test Result

Local build succeeded as expected without above error

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request aims to lazy-load the cv2 module in nemotron_parse.py to prevent import errors in environments where opencv-python is not installed but the model is used for text-only tasks. The change correctly moves the import cv2 statement from the top of the file into the methods that use it.

My review identifies an opportunity to make the lazy loading even more effective. Currently, cv2 is imported during the initialization of NemotronParseImageProcessor, which may still be too early. I've suggested a change to defer the import until an image is actually processed, fully aligning with the PR's goal.

[mergify]

Hi @kiersten-stokes, the pre-commit checks have failed. Please run:

uv pip install pre-commit
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy or markdownlint failing?

mypy and markdownlint are run differently in CI. If the failure is related to either of these checks, please use the following commands to run them locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10
# For markdownlint
pre-commit run --hook-stage manual markdownlint

[mgoin]

Follows the pattern in the result of the codebase, thanks!