Seperate benchmarks by enabled features (#1538)

* Use cfg to seperate benchmark results based on features

* Remove allow pragma

* Avoid some unnecessary returns.

* Improve benchmarks layout

* Derive bench_name from actual device

* Run CPU benchmarks even when GPU feature is enabled

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Co-authored-by: Laurent <laurent.mazare@gmail.com>

candle-core/benches/bench_main.rs

@@ -0,0 +1,4 @@
+mod benchmarks;
+
+use criterion::criterion_main;
+criterion_main!(benchmarks::matmul::benches);