add num_shards to MilvusConfig.to_dict()

[ZebinRen]

In #526 an argument num_shards is added to milvus, however, this argument is not included in the to_dict() function of MilvusConfig, leading to an error at line

VectorDBBench/vectordb_bench/backend/clients/milvus/milvus.py

Line 67 in d6dc089

num_shards=self.db_config.get("num_shards"),

.

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