langchain-huggingface: Make transformers and sentence-transformers packages optional

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Feature request

Make transformers and sentence-transformers packages optional.

Motivation

I would like to use the ChatHuggingFace class through the inference-api. The model will thus be running on HuggingFace servers behind their API. I shouldn't have to install the heavy torch-dependent packages transformers and sentence-transformers to use Hugging Face models through the API.

Proposal (If applicable)

Make transformers and sentence-transformers extra dependencies, installable via pip install langchain-huggingface[transformers]`.