Revert "removing heuristic"

This reverts commit 9a7a84a.

src/gpuarrays.jl

@@ -2,6 +2,27 @@
 
 GPUArrays.device(x::MtlArray) = x.dev
 
+import KernelAbstractions
+import KernelAbstractions: Backend
+
+@inline function GPUArrays.launch_heuristic(::MetalBackend, 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)
+
+    ctx = KA.mkcontext(obj, ndrange, iterspace)
+
+    kernel = @metal launch=false obj.f(ctx, args...)
+
+    # The pipeline state automatically computes occupancy stats
+    threads = min(elements, kernel.pipeline.maxTotalThreadsPerThreadgroup)
+    blocks  = cld(elements, threads)
+
+    return (; threads=Int(threads), blocks=Int(blocks))
+end
+
 const GLOBAL_RNGs = Dict{MTLDevice,GPUArrays.RNG}()
 function GPUArrays.default_rng(::Type{<:MtlArray})
     dev = device()