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-examples/examples/stable-diffusion/embeddings.rs

@@ -0,0 +1,65 @@
+#![allow(dead_code)]
+use candle::{Result, Tensor, D};
+use candle_nn as nn;
+
+#[derive(Debug)]
+pub struct TimestepEmbedding {
+    linear_1: nn::Linear,
+    linear_2: nn::Linear,
+}
+
+impl TimestepEmbedding {
+    // act_fn: "silu"
+    pub fn new(vs: nn::VarBuilder, channel: usize, time_embed_dim: usize) -> Result<Self> {
+        let linear_1 = nn::linear(channel, time_embed_dim, vs.pp("linear_1"))?;
+        let linear_2 = nn::linear(time_embed_dim, time_embed_dim, vs.pp("linear_2"))?;
+        Ok(Self { linear_1, linear_2 })
+    }
+}
+
+impl TimestepEmbedding {
+    pub fn forward(&self, xs: &Tensor) -> Result<Tensor> {
+        let xs = nn::ops::silu(&self.linear_1.forward(xs)?)?;
+        self.linear_2.forward(&xs)
+    }
+}
+
+#[derive(Debug)]
+pub struct Timesteps {
+    num_channels: usize,
+    flip_sin_to_cos: bool,
+    downscale_freq_shift: f64,
+}
+
+impl Timesteps {
+    pub fn new(num_channels: usize, flip_sin_to_cos: bool, downscale_freq_shift: f64) -> Self {
+        Self {
+            num_channels,
+            flip_sin_to_cos,
+            downscale_freq_shift,
+        }
+    }
+}
+
+impl Timesteps {
+    pub fn forward(&self, xs: &Tensor) -> Result<Tensor> {
+        let half_dim = (self.num_channels / 2) as u32;
+        let exponent =
+            (Tensor::arange(0, half_dim, xs.device())?.to_dtype(xs.dtype())? * -f64::ln(10000.))?;
+        let exponent = (exponent / (half_dim as f64 - self.downscale_freq_shift))?;
+        let emb = exponent.exp()?;
+        // emb = timesteps[:, None].float() * emb[None, :]
+        let emb = (xs.unsqueeze(D::Minus1)? * emb.unsqueeze(0)?)?;
+        let (cos, sin) = (emb.cos()?, emb.sin()?);
+        let emb = if self.flip_sin_to_cos {
+            Tensor::cat(&[&cos, &sin], D::Minus1)?
+        } else {
+            Tensor::cat(&[&sin, &cos], D::Minus1)?
+        };
+        if self.num_channels % 2 == 1 {
+            crate::utils::pad(&emb) // ([0, 1, 0, 0], 'constant', None)
+        } else {
+            Ok(emb)
+        }
+    }
+}