Enabled Azure Active Directory token-based auth access to OpenAI completions (#6313)

With AzureOpenAI openai_api_type defaulted to "azure" the logic in
utils' get_from_dict_or_env() function triggered by the root validator
never looks to environment for the user's runtime openai_api_type
values. This inhibits folks using token-based auth, or really any auth
model other than "azure."

By removing the "default" value, this allows environment variables to be
pulled at runtime for the openai_api_type and thus enables the other
api_types which are expected to work.

---------

Co-authored-by: Ebo <mebstyne@microsoft.com>
Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/llms/openai.py

@@ -617,7 +617,7 @@ class AzureOpenAI(BaseOpenAI):
 
     deployment_name: str = ""
     """Deployment name to use."""
-    openai_api_type: str = "azure"
+    openai_api_type: str = ""
     openai_api_version: str = ""
 
     @root_validator()
@@ -628,9 +628,7 @@ class AzureOpenAI(BaseOpenAI):
             "OPENAI_API_VERSION",
         )
         values["openai_api_type"] = get_from_dict_or_env(
-            values,
-            "openai_api_type",
-            "OPENAI_API_TYPE",
+            values, "openai_api_type", "OPENAI_API_TYPE", "azure"
         )
         return values