Add query function for if we are executing on a GPU

[vchuravy]

Requires an API bump since backend need to implement this
with device_overlay.

[maleadt]

Feels a little ad-hoc. I haven't thought this through, but what about a Device or Accelerator object, which could be used to query additional properties (SIMD width, etc)? The GPU-ness could be a part of the type hierarchy (CUDAGPU <: AbstractGPU <: AbstractAccelerator or whatever).

[vchuravy]

Hm yeah, I was thinking current_backend() but Simon wanted the ability to not use it on the top-level.

We could pre-empt some other work I am thinking about and expose something like target() or compilation_target() and and then return a Features struct.

[simonbyrne]

We could pre-empt some other work I am thinking about and expose something like target() or compilation_target() and and then return a Features struct.

Any more thoughts on this?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.96%. Comparing base (3165d35) to head (852508b).
Report is 107 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cpu.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
- Coverage   83.10%   82.96%   -0.15%     
==========================================
  Files           7        9       +2     
  Lines         580      628      +48     
==========================================
+ Hits          482      521      +39     
- Misses         98      107       +9     

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