Update RMM memory resource APIs to ref-based equivalents

[bdice]

Summary

• Replace pointer-based RMM APIs (which will soon be deprecated) with ref-based alternatives as part of the RMM CCCL 3.2 memory resource migration
• Update device_resources_manager.hpp: set_current_device_resource() → set_current_device_resource_ref()
• Update benchmark.hpp: Update using_pool_memory_res to use ref-based APIs
• Update gather.cu: Use ref-based APIs for memory resource management
• Update subsample.cu: Use ref-based APIs for memory resource management
• Update buffer.cpp: set_current_device_resource() → set_current_device_resource_ref()