duplicate validator function

[Almenon]

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
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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.

[Almenon]

Fix and workaround described in #439