refactor: use extra_body to pass in input_type params for asymmetric embedding models for NVIDIA Inference Provider

[jiayin-nvidia]

What does this PR do?

Previously, the NVIDIA inference provider implemented a custom openai_embeddings method with a hardcoded input_type="query" parameter, which is required by NVIDIA asymmetric embedding models(#3205).
Recently extra_body parameter is added to the embeddings API (#3794). So, this PR updates the NVIDIA inference provider to use the base OpenAIMixin.openai_embeddings method instead and pass the input_type through the extra_body parameter for asymmetric embedding models.

Test Plan

Run the following command for the embedding_model: nvidia/llama-3.2-nv-embedqa-1b-v2, nvidia/nv-embedqa-e5-v5, nvidia/nv-embedqa-mistral-7b-v2, and snowflake/arctic-embed-l.

pytest -s -v tests/integration/inference/test_openai_embeddings.py --stack-config="inference=nvidia" --embedding-model={embedding_model} --env NVIDIA_API_KEY={nvidia_api_key} --env NVIDIA_BASE_URL="https://integrate.api.nvidia.com" --inference-mode=record

[ashwinb]

you can always add extra body unconditionally? if it is null, it is null no harm made. you don't need the double check?

[jiayin-nvidia]

Good point! I also consolidated the verification and retrieval of extra_body for asymmetric models into a single helper function that's used consistently across all openai_embeddings test cases.

[ashwinb]

nah I don't think we need to be that defensive. i'd just simplify this a bunch more. simply make the callsites be:

client.embeddings.create(..., extra_body=get_extra_body())

that's it!

[ashwinb]

where is this used now?

[jiayin-nvidia]

Update to put it in get_extra_body_for_model: if it is not an asymmetric model, return None for the extra_body.