Expose the cudnn algo in the conv ops. (#2892)

* Set the algo.

* Expose the cudnn preferred algo for conv ops.

candle-examples/examples/yolo-v8/model.rs

@@ -92,6 +92,7 @@ impl ConvBlock {
             stride,
             groups: 1,
             dilation: 1,
+            cudnn_fwd_algo: None,
         };
         let bn = batch_norm(c2, 1e-3, vb.pp("bn"))?;
         let conv = conv2d_no_bias(c1, c2, k, cfg, vb.pp("conv"))?.absorb_bn(&bn)?;