Metal Unary: Add benchmarks and process kernels in a tile based fashion (#2056)

* add basic unary bench for sqrt

* process unary commands in tiles of 4

* re-enable all benchmarks

* rename helper to unary

* modify approach to split up tiled and non-tiled operations

* undo bench ignore for other tests

* update tile size to 2

* only perform the optimization on the contiguous even numbered element case

candle-core/benches/bench_main.rs

@@ -8,4 +8,5 @@ criterion_main!(
     benchmarks::where_cond::benches,
     benchmarks::conv_transpose2d::benches,
     benchmarks::qmatmul::benches,
+    benchmarks::unary::benches
 );