Default to base_url if provided

[Wauplin]

Fix #2804.

base_url was first introduced to match OpenAI chat completion API. It has the exact same behavior as self.model when using client.chat_completion. However, since it's a public parameter also for other tasks, I do think it makes sense to use it if provided. The problem in #2804 was that user provided a URL as base_url and expected it to be used in text_generation (which seems fair) but wasn't. Therefore it defaults to Inference API with default model which in not intended. This small PR should prevent this.

---

Not related at all but I took the opportunity to rephrase a bit the error raised in case of a 422 unprocessable entity. Usually the response body provides information about what's missing/not valid. Let's add it to the error message as a string.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[hanouticelina]

thanks for adding more context in the error message using the response body, it was a bit annoying to parse manually the response to know where the error is coming from 😄 I was going to open a PR for that too, so perfect! 👌