docs: langchain-openai (#15513)

Updates docs and cookbooks to import ChatOpenAI, OpenAI, and OpenAI
Embeddings from `langchain_openai`

There are likely more

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Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

cookbook/tree_of_thought.ipynb

@@ -24,7 +24,7 @@
     }
    ],
    "source": [
-    "from langchain_community.llms import OpenAI\n",
+    "from langchain_openai import OpenAI\n",
     "\n",
     "llm = OpenAI(temperature=1, max_tokens=512, model=\"gpt-3.5-turbo-instruct\")"
    ]