Revert "removing heuristic"

This reverts commit 54796ad.

src/gpuarrays.jl

@@ -1,12 +1,33 @@
 # GPUArrays.jl interface
 
+import KernelAbstractions
+import KernelAbstractions: Backend
+
 #
 # Device functionality
 #
 
 
 ## execution
 
+@inline function GPUArrays.launch_heuristic(::oneAPIBackend, obj::O, args::Vararg{Any,N};
+                                            elements::Int, elements_per_thread::Int) where {O,N}
+    ndrange = ceil(Int, elements / elements_per_thread)
+    ndrange, workgroupsize, iterspace, dynamic = KA.launch_config(obj, ndrange,
+                                                                  nothing)
+
+    # this might not be the final context, since we may tune the workgroupsize
+    ctx = KA.mkcontext(obj, ndrange, iterspace)
+
+    kernel = @oneapi launch=false obj.f(ctx, args...)
+
+    items = launch_configuration(kernel)
+    # XXX: how many groups is a good number? the API doesn't tell us.
+    #      measured on a low-end IGP, 32 blocks seems like a good sweet spot.
+    #      note that this only matters for grid-stride kernels, like broadcast.
+    return (threads=items, blocks=32)
+end
+
 const GLOBAL_RNGs = Dict{ZeDevice,GPUArrays.RNG}()
 function GPUArrays.default_rng(::Type{<:oneArray})
     dev = device()