Metal: f16 and bf16 where_cond + benchmark (#1545)

* Use cfg to seperate benchmark results based on features

* Add metal where_cond for f16 and bf16. Add benchmark

* Remove allow pragma

* Avoid some unnecessary returns.

* Improve benchmarks layout

* Updated feature separated benchmarks

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

candle-core/benches/benchmarks/mod.rs

@@ -1,4 +1,5 @@
 pub(crate) mod matmul;
+pub(crate) mod where_cond;
 
 use candle_core::{Device, Result};