Fix Issue #6650: Enable Azure Active Directory token-based auth access for AzureChatOpenAI (#8622)

When using AzureChatOpenAI the openai_api_type defaults to "azure". The
utils' get_from_dict_or_env() function triggered by the root validator
does not look for user provided values from environment variables
OPENAI_API_TYPE, so other values like "azure_ad" are replaced with
"azure". This does not allow the use of token-based auth.

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.

This fixes #6650

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/chat_models/azure_openai.py

@@ -45,7 +45,7 @@ class AzureChatOpenAI(ChatOpenAI):
     """
 
     deployment_name: str = ""
-    openai_api_type: str = "azure"
+    openai_api_type: str = ""
     openai_api_base: str = ""
     openai_api_version: str = ""
     openai_api_key: str = ""
@@ -71,9 +71,7 @@ class AzureChatOpenAI(ChatOpenAI):
             "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", default="azure"
         )
         values["openai_organization"] = get_from_dict_or_env(
             values,