feat: add LangChainGo Huggingface backend (#446)

Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

examples/langchain-huggingface/README.md

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+# Data query example
+
+Example of integration with HuggingFace Inference API with help of [langchaingo](https://github.com/tmc/langchaingo).
+
+## Setup
+
+Download the LocalAI and start the API:
+
+```bash
+# Clone LocalAI
+git clone https://github.com/go-skynet/LocalAI
+
+cd LocalAI/examples/langchain-huggingface
+
+docker-compose up -d
+```
+
+Node: Ensure you've set `HUGGINGFACEHUB_API_TOKEN` environment variable, you can generate it
+on [Settings / Access Tokens](https://huggingface.co/settings/tokens) page of HuggingFace site.
+
+This is an example `.env` file for LocalAI:
+
+```ini
+MODELS_PATH=/models
+CONTEXT_SIZE=512
+HUGGINGFACEHUB_API_TOKEN=hg_123456
+```
+
+## Using remote models
+
+Now you can use any remote models available via HuggingFace API, for example let's enable using of
+[gpt2](https://huggingface.co/gpt2) model in `gpt-3.5-turbo.yaml` config:
+
+```yml
+name: gpt-3.5-turbo
+parameters:
+  model: gpt2
+  top_k: 80
+  temperature: 0.2
+  top_p: 0.7
+context_size: 1024
+backend: "langchain-huggingface"
+stopwords:
+- "HUMAN:"
+- "GPT:"
+roles:
+  user: " "
+  system: " "
+template:
+  completion: completion
+  chat: gpt4all
+```
+
+Here is you can see in field `parameters.model` equal `gpt2` and `backend` equal `langchain-huggingface`.
+
+## How to use
+
+```shell
+# Now API is accessible at localhost:8080
+curl http://localhost:8080/v1/models
+# {"object":"list","data":[{"id":"gpt-3.5-turbo","object":"model"}]}
+
+curl http://localhost:8080/v1/completions -H "Content-Type: application/json" -d '{
+  "model": "gpt-3.5-turbo",
+  "prompt": "A long time ago in a galaxy far, far away",
+  "temperature": 0.7
+}'
+```

examples/langchain-huggingface/docker-compose.yml

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+version: '3.6'
+
+services:
+  api:
+    image: quay.io/go-skynet/local-ai:latest
+    build:
+      context: ../../
+      dockerfile: Dockerfile
+    ports:
+      - 8080:8080
+    env_file:
+      - ../../.env
+    volumes:
+      - ./models:/models:cached
+    command: ["/usr/bin/local-ai"]

examples/langchain-huggingface/models/completion.tmpl

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+{{.Input}}

examples/langchain-huggingface/models/gpt-3.5-turbo.yaml

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+name: gpt-3.5-turbo
+parameters:
+  model: gpt2
+  top_k: 80
+  temperature: 0.2
+  top_p: 0.7
+context_size: 1024
+backend: "langchain-huggingface"
+stopwords:
+- "HUMAN:"
+- "GPT:"
+roles:
+  user: " "
+  system: " "
+template:
+  completion: completion
+  chat: gpt4all

examples/langchain-huggingface/models/gpt4all.tmpl

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+The prompt below is a question to answer, a task to complete, or a conversation to respond to; decide which and write an appropriate response.
+### Prompt:
+{{.Input}}
+### Response: