Add device_async_resource_ref convenience alias

[harrism]

Description

Adds a new header include/rmm/resource_ref.hpp that currently just contains a single using alias to make it easier to spell a device-accessible async_resource_ref.

Closes #1442

rmm::device_async_resource_ref is an alias for cuda::mr::async_resource_ref<cuda::mr::device_accessible>.

This is 30 characters vs. 57. It is shorter than the currently used resource pointer spelling: rmm::mr::device_memory_resource*.

Also updates tests to use this alias.

Also makes one unrelated doc change, I noticed pinned_host_memory_resource was not added to a doxygen group, so I fixed that.

I was going to put this in cuDF, but @bdice suggested maybe it would be useful to have it in RMM.

[bdice]

Want to add a test?

[jrhemstad]

This is 34 characters vs. 57. I would like to make it shorter, but I think it would lose some information.

You could probably drop the "async" since RMM's allocation APIs have always been async and it's well understood at this point.

So rmm::mr::device_resource_ref.

It wouldn't hurt anything to drop the mr:: either.

[harrism]

This is 34 characters vs. 57. I would like to make it shorter, but I think it would lose some information.

You could probably drop the "async" since RMM's allocation APIs have always been async and it's well understood at this point.

So rmm::mr::device_resource_ref.

It wouldn't hurt anything to drop the mr:: either.

RMM has always had some MRs that are synchronous (e.g. cuda_memory_resource). That's what supports_streams() was for. Also host_memory_resource is synchronous. The fact that nobody has used anything non-stream-ordered from RMM so far is a different fact.

If I remove mr::, should I also move the file up a level?

[harrism]

Answered my own question. Moved the header up a level and move up to rmm:: namespace. This is now shorter spelling than the resource pointer used by RAPIDS libraries, so it's a net win:

rmm::device_async_resource_ref mr; // new
rmm::mr::device_memory_resource *mr; // old

[harrism]

/merge