[Bug fix]: Fix issue of OPENAI_API_BASE env variable being mandatory

[deshraj]

Description

User reported on slack that the new version has a regression resulting into the following error:

>>> from embedchain import App
>>> app = App()
>>> app.add("http://deshraj.xyz")
Doc content has not changed. Skipping creating chunks and embeddings
'0d8813f767d2cf9bf728bcc44fc55ff0'
>>> app.chat("Who is Deshraj?")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/deshrajyadav/Projects/embedchain/embedchain/embedchain.py", line 576, in chat
    answer = self.llm.chat(
             ^^^^^^^^^^^^^^
  File "/Users/deshrajyadav/Projects/embedchain/embedchain/llm/base.py", line 278, in chat
    answer = self.get_answer_from_llm(prompt)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/deshrajyadav/Projects/embedchain/embedchain/llm/base.py", line 143, in get_answer_from_llm
    return self.get_llm_model_answer(prompt)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/deshrajyadav/Projects/embedchain/embedchain/llm/openai.py", line 27, in get_llm_model_answer
    response = self._get_answer(prompt, self.config)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/deshrajyadav/Projects/embedchain/embedchain/llm/openai.py", line 42, in _get_answer
    base_url = config.base_url or os.environ["OPENAI_API_BASE"]
                                  ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "<frozen os>", line 679, in __getitem__
KeyError: 'OPENAI_API_BASE'

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Unit Test