huggingface[fix]: HuggingFaceEndpointEmbeddings model parameter passing error when async embed

[vegetablest]

This change refines the handling of _model_kwargs in POST requests. Instead of nesting _model_kwargs as a dictionary under the parameters key, it is now directly unpacked and merged into the request's JSON payload. This ensures that the model parameters are passed correctly and avoids unnecessary nesting.E. g.:

import asyncio

from langchain_huggingface.embeddings import HuggingFaceEndpointEmbeddings

embedding_input = ["This input will get multiplied" * 10000]

embeddings = HuggingFaceEndpointEmbeddings(
    model="http://127.0.0.1:8081/embed",
    model_kwargs={"truncate": True},
)

# Truncated parameters in synchronized methods are handled correctly
embeddings.embed_documents(texts=embedding_input)
# The truncate parameter is not handled correctly in the asynchronous method,
# and 413 Request Entity Too Large is returned.
asyncio.run(embeddings.aembed_documents(texts=embedding_input))

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Name	Status	Preview	Comments	Updated (UTC)
langchain	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 15, 2024 2:00am

[vegetablest]

@efriis, would you mind reviewing this PR? Please let me know if there’s anything I should clarify.