Unified memory allocations

[christiangnrd]

Close #601

@vchuravy The idea is that you request unified if you want to uallocate some memory with unified memory to speed up operations that necessitate CPU access (like 1d reduce result or the indices container in findall). Otherwise just use the default storage type. Backends would have to add support for the unified kwarg.

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[vchuravy]

Perhaps we can make this backwards portable and add it to 0.9 first?

[vchuravy]

Thanks once we have it merged in 0.9, please update this PR so that the implementations match.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 51.24%. Comparing base (1ac546f) to head (98ba890).

Files with missing lines	Patch %	Lines
src/KernelAbstractions.jl	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
+ Coverage   47.54%   51.24%   +3.70%     
==========================================
  Files          22       21       -1     
  Lines        1729     1610     -119     
==========================================
+ Hits          822      825       +3     
+ Misses        907      785     -122     

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[vchuravy]

ready to merge?

[christiangnrd]

@vchuravy Yes ready to merge. Only thing I want to make sure you didn't miss is that contrary to the 0.9 PR, this has supports_unified as "must". I you would rather it still say "should" commit the suggestion. Either way merge away!

Suggested change

	Backend implementations **must** implement this function
	Backend implementations **should** implement this function