Fix pyright breakage when vllm and llama_cpp are not installed

[taenin]

Description

Fixes broken precommits.

Currently they are broken when vllm and llama_cpp are not installed:

check for added large files..............................................Passed
check python ast.........................................................Passed
check for case conflicts.................................................Passed
check docstring is first.................................................Passed
check that executables have shebangs.................(no files to check)Skipped
check json...........................................(no files to check)Skipped
check for merge conflicts................................................Passed
check that scripts with shebangs are executable..........................Passed
check toml...........................................(no files to check)Skipped
check yaml...........................................(no files to check)Skipped
debug statements (python)................................................Passed
detect private key.......................................................Passed
fix end of files.........................................................Passed
fix utf-8 byte order marker..............................................Passed
mixed line ending........................................................Passed
python tests naming......................................................Passed
don't commit to branch...................................................Passed
trim trailing whitespace.................................................Passed
ruff.....................................................................Passed
ruff-format..............................................................Passed
pyright..................................................................Failed
- hook id: pyright
- exit code: 1

/Users/matthewpersons/Repos/oumi/tests/unit/inference/test_inference_engine_init.py
  /Users/matthewpersons/Repos/oumi/tests/unit/inference/test_inference_engine_init.py:33:12 - error: Import "vllm" could not be resolved (reportMissingImports)

Related issues

Fixes N/A

Before submitting

• This PR only changes documentation. (You can ignore the following checks in that case)
• Did you read the contributor guideline Pull Request guidelines?
• Did you link the issue(s) related to this PR in the section above?
• Did you add / update tests where needed?

Reviewers

At least one review from a member of oumi-ai/oumi-staff is required.