duplicate validator function

**Describe the bug**
```
pydantic.errors.ConfigError: duplicate validator function "langchain.prompts.base.BasePromptTemplate.validate_variable_names"; if this is intended, set `allow_reuse=True`
```

**To Reproduce**
Steps to reproduce the behavior:
1. `from langchain.llms import OpenAI`
2. Run once
3. Run again

**Expected behavior**
No error

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Other Information (please complete the following information):**
- OS: [e.g. Windows10]
- Python Version [e.g. 22]

**Additional context**
Add any other context about the problem here.