Add a stable diffusion example (#328)

* Start adding a stable-diffusion example.

* Proper computation of the causal mask.

* Add the chunk operation.

* Work in progress: port the attention module.

* Add some dummy modules for conv2d and group-norm, get the attention module to compile.

* Re-enable the 2d convolution.

* Add the embeddings module.

* Add the resnet module.

* Add the unet blocks.

* Add the unet.

* And add the variational auto-encoder.

* Use the pad function from utils.

candle-nn/src/ops.rs

@@ -32,3 +32,7 @@ pub fn log_softmax<D: candle::shape::Dim>(xs: &Tensor, d: D) -> Result<Tensor> {
     let log_sm = diff.broadcast_sub(&sum_exp.log()?)?;
     Ok(log_sm)
 }
+
+pub fn silu(xs: &Tensor) -> Result<Tensor> {
+    xs / (xs.neg()?.exp()? + 1.0)?
+}