Tracing for the phi model (#936)

* Add some tracing bits to mixformers.

* Add the missing file.

* Add the conv2d layer to with-tracing.

* Improve the tracing usage.

candle-transformers/src/models/stable_diffusion/resnet.rs

@@ -4,7 +4,7 @@
 //!
 //! Denoising Diffusion Implicit Models, K. He and al, 2015.
 //! https://arxiv.org/abs/1512.03385
-use super::utils::{conv2d, Conv2d};
+use crate::models::with_tracing::{conv2d, Conv2d};
 use candle::{Result, Tensor, D};
 use candle_nn as nn;
 use candle_nn::Module;