community: FlashrankRerank support loading customer client (#23350)

Description: FlashrankRerank Document compressor support loading
customer client
Issue: #23338

Co-authored-by: gongwn1 <gongwn1@lenovo.com>

libs/community/langchain_community/document_compressors/flashrank_rerank.py

@@ -38,6 +38,9 @@ class FlashrankRerank(BaseDocumentCompressor):
     @root_validator(pre=True)
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
+        if "client" in values:
+            return values
+        else:
             try:
                 from flashrank import Ranker
             except ImportError: