[Tools] change max_tokens default value to 512

[melionel]

This pull request includes a single change to the openai_gpt4v.yaml file in the promptflow-tools package. The default value for max_tokens has been updated to 512.

• src/promptflow-tools/promptflow/tools/yamls/openai_gpt4v.yaml: Updated the default value for max_tokens to 512.# Description

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