Deprecate and remove get_mem_info methods from memory resources.

[harrism]

device_memory_resource::get_mem_info has always been hard to support for arbitrary memory resources. Sometimes it is impossible to calculate and so MRs have another method supports_get_mem_info() that returns true if get_mem_info() returns useful values.

We are now in the process of adopting cuda::memory_resource and resource_ref concepts into RMM, and we would like to embrace the flexibility and composability they enable. cuda::memory_resource does not provide anything like get_mem_info(), just like its inspiration std::pmr::memory_resource.

I am now experimenting with defining a host_pinned_memory_resource using the cuda::memory_resource concept and not deriving from rmm::mr::device_memory_resource. This works as an upstream for other resources, such as pool_memory_resource, except for one detail: pool_memory_resource relies on cudaMemGetInfo if the initial and maximum pool sizes are not specified, but that function cannot be used for pinned host memory!

I propose that we deprecate and remove the use of supports_get_mem_info() and get_mem_info(). For resources like pool, I recommend that we change them to NOT query memory sizes and make initial and maximum pool sizes required constructor parameters. While this may seem to make them harder to use, we can expose rmm::detail::available_device_memory() (which calls cudaMemGetInfo) as a utility in the public interface to use when the user knows the pool is device memory. We may also be able to provide a utility function that estimates host memory available.

Once we move entirely to the cuda::memory_resource concept, we will be able to drop the device_memory_resource base class and in that way remove the deprecated get_mem_info methods.

See also #305

Tasks

Beta Give feedback

1. Change pool_memory_resource to require explicit initial and maximum pool sizes. Possibly allow a default of zero for initial size. #1416
   cpp improvement +2
   
2. Make rmm::mr::device_memory_resource::get_mem_info() and rmm::mr::device_memory_resource::supports_get_mem_info() not pure virtual, remove derived implementations and all calls in RMM code including tests. #1426
   cpp feature request +2
   
3. Deprecate rmm::mr::device_memory_resource::get_mem_info() and rmm::mr::device_memory_resource::supports_get_mem_info(). #1427
   cpp feature request +2
   
4. Remove deprecated rmm::mr::device_memory_resource_get_mem_info() and rmm::mr::device_memory_resource::supports_get_mem_info(). #1428
   cpp feature request +2