[SYCL][DOC] Add initial spec draft for new default device

[jbrodman]

This is a draft for a new extension to add the notion of a default device. It adds a method to query the current device from the calling thread as well as a method to change it.

[etomzak]

The default device portion of this extension is very interesting for SYCL SC. We probably need something similar as a core feature for SYCL SC 1.0.

I might be missing something, but I don't immediately see how the default device, memory & allocation, and submission portions of this extension are related. Are they? I ask because default device solves some problems for SYCL SC, so it would be good to align with SYCL on solving that problem. It's less clear how relevant the other parts of the extension are to SYCL SC.

[gmlueck]

I've added some granular comments below, but I have a big picture comment too.

What is the customer request that we are solving with this? I wonder if we are going to far by adding the concept of the default queue and the functions that implicitly submit work to the default queue. In particular the memory functions seem like they would be confusing. If you saw code like this:

sycl::copy(src, dest, N);

I think most people would expect this to immediately copy memory from src to dest, but that is not the semantic of this command. It seems like this confusion would outweigh whatever benefit we gain. Even a call to barrier seems like it could be confusing:

sycl::barrier();

I can imagine that people would expect this to be some sort of host barrier instruction.

If we do not have a request for these functions that assume a default queue, I think it would be better to wait until there is a more clear requirement. We might want to consider new names if we ever do add functions like this.

Is the real user request just set_current_device and get_current_device? If so, maybe we should just add those for now.

[gmlueck]

@jbrodman let me know when this is ready to review again. I think there are still some spelling / formatting comments to address.

[dm-vodopyanov]

From the description: "It also extends the new launch mechanisms with queue-less forms that utilize the default device's default queue."

Am I correct that these changes will be introduced in separate PR/spec?

[gmlueck]

Good catch. The PR description should be changed.

[jbrodman]

Fixed. Took a second to realize this was the PR description and not the spec. :)