Fix: CoHereRerank not respecting base_url when provided

[hekmon]

What problem does this PR solve?

vLLM provider with a reranking model does not work : as vLLM uses under the hood the CoHereRerank provider with a base_url, if this URL is not passed to the Cohere client any attempt will endup on the Cohere SaaS (sending your private api key in the process) instead of your vLLM instance.

Type of change

• [x] Bug Fix (non-breaking change which fixes an issue)
• [ ] New Feature (non-breaking change which adds functionality)
• [ ] Documentation Update
• [ ] Refactoring
• [ ] Performance Improvement
• [ ] Other (please describe):