Added docstrings and unit tests for `core.multimodal` (#16872)

...x-core/llama_index/core/multi_modal_llms/generic_utils.py

llama-index-core/poetry.lock

llama-index-core/pyproject.toml

llama-index-core/tests/multi_modal_llms/BUILD

...ts/multi_modal_llms/test_base_multi_modal_llm_metadata.py

...a-index-core/tests/multi_modal_llms/test_generic_utils.py