Use modern OpenAI endpoints for embeddings (#6573)

- Description: 

LangChain passes
[engine](https://github.com/hwchase17/langchain/blob/master/langchain/embeddings/openai.py#L256)
and not `model` as a field when making OpenAI requests. Within the
`openai` Python library, for OpenAI requests, this [makes a
call](https://github.com/openai/openai-python/blob/main/openai/api_resources/abstract/engine_api_resource.py#L58)
to an endpoint of the form
`https://api.openai.com/v1/engines/{engine_id}/embeddings`.

These endpoints are
[deprecated](https://help.openai.com/en/articles/6283125-what-happened-to-engines)
in favor of endpoints of the format
`https://api.openai.com/v1/embeddings`, where `model` is passed as a
parameter in the request body.

While these deprecated endpoints continue to function for now, they may
not be supported indefinitely and should be avoided in favor of the
newer API format.

It appears that `engine` was passed in instead of `model` to make both
Azure OpenAI and OpenAI calls work similarly. However, the inclusion of
`engine`
[causes](https://github.com/openai/openai-python/blob/main/openai/api_resources/abstract/engine_api_resource.py#L58)
OpenAI to use the deprecated endpoint, requiring a diverging code path
for Azure OpenAI calls where `engine` is passed in additionally (Azure
OpenAI requires `engine` to specify a deployment, and can optionally
take in `model`).

In the long-term, it may be worth considering spinning off Azure OpenAI
embeddings into a separate class for ease of use and maintenance,
similar to the [implementation for chat
models](https://github.com/hwchase17/langchain/blob/master/langchain/chat_models/azure_openai.py).

langchain/embeddings/openai.py

@@ -253,7 +253,7 @@ class OpenAIEmbeddings(BaseModel, Embeddings):
     @property
     def _invocation_params(self) -> Dict:
         openai_args = {
-            "engine": self.deployment,
+            "model": self.model,
             "request_timeout": self.request_timeout,
             "headers": self.headers,
             "api_key": self.openai_api_key,
@@ -262,6 +262,8 @@ class OpenAIEmbeddings(BaseModel, Embeddings):
             "api_type": self.openai_api_type,
             "api_version": self.openai_api_version,
         }
+        if self.openai_api_type in ("azure", "azure_ad", "azuread"):
+            openai_args["engine"] = self.deployment
         if self.openai_proxy:
             import openai