Pydantic version conflict with vLLM

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In short, vLLM depends on pydantic >= 2,
```
pydantic >= 2.0 # Required for OpenAI server.
```
on the other hand, `fastchat/serve/openai_api_server.py` depends on v1.x
```python
try:
from pydantic.v1 import BaseSettings
except ImportError:
from pydantic import BaseSettings
```

I installed the dependencies by `pip install fschat[model_worker] vllm`, it causes error at runtime:
```
Traceback (most recent call last):

File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main

return _run_code(code, main_globals, None,

File "/usr/lib/python3.10/runpy.py", line 86, in _run_code

exec(code, run_globals)

File "/usr/local/lib/python3.10/dist-packages/fastchat/serve/openai_api_server.py", line 24, in

from pydantic import BaseSettings

File "/usr/local/lib/python3.10/dist-packages/pydantic/__init__.py", line 374, in __getattr__

return _getattr_migration(attr_name)

File "/usr/local/lib/python3.10/dist-packages/pydantic/_migration.py", line 296, in wrapper

raise PydanticImportError(

pydantic.errors.PydanticImportError: `BaseSettings` has been moved to the `pydantic-settings` package. See https://docs.pydantic.dev/2.6/migration/#basesettings-has-moved-to-pydantic-settings for more details.

```

Start with fastchat/serve/openai_api_server.py and reproduce the failure using the reported `pip install fschat[model_worker] vllm` command. Trace the BaseSettings import and dependency versions; done means the OpenAI API server starts successfully with vLLM installed without the Pydantic import error.

Written by the indexing model from the issue text.