Add support for conv_transpose2d on Metal backend (#1903)

* add support for conv transpose 2d and add bench mark for float types

* update bench calculation

* enable testing all conv operations on metal

candle-core/benches/bench_main.rs

@@ -5,5 +5,6 @@ criterion_main!(
     benchmarks::affine::benches,
     benchmarks::matmul::benches,
     benchmarks::random::benches,
-    benchmarks::where_cond::benches
+    benchmarks::where_cond::benches,
+    benchmarks::conv_transpose2d::benches,
 );