Fix get_pool_memory_resource return type

[tfeher]

The return type of get_pool_memory_resource was changed in #1469 from pool_memory_resource to device_memory_resource. There are debug logs in the code (example), which query the pool size, that would fail when debug logging is enabled. This PR restores the output type of get_pool_memory_resource.

[tfeher]

@ahendriksen was it just an oversight to change the return type, or is there a reason (compile time) for such change? In the latter case we could just remove the printouts (longer term we consider removing such pools anyway).

[ahendriksen]

The return type of get_pool_memory_resource was changed specifically to avoid the inclusion of the pool_memory_resource.hpp header, which is one of the few RMM headers which unconditionally includes spdlog. Including spdlog slows down compile times considerably.

The memory_pool.hpp file is included from cudart_utils.cuh, so this PR in current state would undo all compile time gains from splitting out memory_pool-inl.hpp.

As an alternative, would it be possible to dynamically cast to pool_memory_resource in those places where the debug logging takes place?

[tfeher]

Thanks @ahendriksen for the explanation. I am closing this in favor of #1484.