Make the trailing / optional at openai.base_url setting

[kylehh]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Use self._base_url to replace self.base_url in BaseClient class's _prepare_url method.

Additional context & links

It does not hurt OpenAI class client, because self.base_url is referred to self._base_url base on this line

But this change would help the module_client cases, when we are setting
openai.base_url="an_endpoint_url_WITHOUT_training_/_at_the_end"
Due to this line, the base_url would directly return the URL WITHOUT trailing / and fail the requst
The workaround is always add training / at the end of the module client base_url, but with this PR fix, this step is optional (_base_url is / padded already here)

[kylehh]

Here is how to reproduce the problem. Following code would fail without the trailing /

openai.base_url = "https://api.openai.com/v1"
# Instead, you have to add the trailing /
#openai.base_url = "https://api.openai.com/v1/"
openai.api_key = "sk-xxxx"
chat_completion = openai.chat.completions.create()

[rattrayalex]

cc @RobertCraigie can you take a look at this on Monday?

[chrishalbert]

Can you add tests to the test_client.py to address the possible scenarios? Otherwise, it looks good to me.

[kylehh]

base_url is the testing parameters get from "base_url = os.environ.get("TEST_API_BASE_URL", "http://127.0.0.1:4010")"
With this fix, ppl should be able to work with URL with and without trailing '/'. Not sure how to include in the unit test for this scenaries

[rattrayalex]

gentle ping @RobertCraigie