add langchain examples

examples/langchain/README.md

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+# LangChain
+
+This example is a basic "hello world" of using LangChain with Ollama.
+
+## Setup
+
+```
+pip install -r requirements.txt
+```
+
+## Run
+
+```
+python main.py
+```

examples/langchain/main.py

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+from langchain.llms import Ollama
+llm = Ollama(model="llama2")
+res = llm.predict("hi!")
+print (res)

examples/langchain/requirements.txt

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+langchain==0.0.259