docs: langchain-openai (#15513)

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

There are likely more

---------

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

cookbook/selecting_llms_based_on_context_length.ipynb

@@ -18,9 +18,9 @@
    "outputs": [],
    "source": [
     "from langchain.prompts import PromptTemplate\n",
-    "from langchain_community.chat_models import ChatOpenAI\n",
     "from langchain_core.output_parsers import StrOutputParser\n",
-    "from langchain_core.prompt_values import PromptValue"
+    "from langchain_core.prompt_values import PromptValue\n",
+    "from langchain_openai import ChatOpenAI"
    ]
   },
   {