Reflecting the growing consolidation wave sparked by healthcare reform, kidney dialysis giant DaVita Inc. has agreed to acquire Torrance-based HealthCare Partners, the largest operator of medical groups in the U.S., for $4.42 billion in cash and stock. The deal represents the latest sign of insurers, hospitals and other large healthcare companies buying up physician practices in order to better position themselves for changes in how the federal government is reimbursing for medical care.