Improve reduce perf and add contiguous impl

candle-core/benches/benchmarks/reduce.rs

@@ -0,0 +1,36 @@
+use candle_core::{DType, Tensor};
+use criterion::{black_box, criterion_group, Criterion, Throughput};
+use std::time::Instant;
+use crate::benchmarks::{bench_name, device, BenchDevice};
+
+fn run(a: &Tensor) {
+    a.sum(2).unwrap();
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let b = 1;
+    let m = 2048;
+    let k = 2048;
+
+    let device = device().unwrap();
+
+    let a = Tensor::rand(-1000.0f32, 1000.0f32, (b, m, k), &device).unwrap();
+
+    let flops = b * m * k * DType::F32.size_in_bytes();
+
+    let mut group = c.benchmark_group(bench_name("reduce"));
+    group.throughput(Throughput::Bytes(flops as u64));
+    group.bench_function("iter", move |b| {
+        b.iter_custom(|iters| {
+            let start = Instant::now();
+            for _i in 0..iters {
+                run(black_box(&a));
+            }
+            device.sync().unwrap();
+            start.elapsed()
+        })
+    });
+    group.finish();
+}
+
+criterion_group!(benches, criterion_benchmark);