Add Euler Ancestral Discrete Scheduler (#1390)

* Add Euler Ancestral Discrete Scheduler

* Fix a bug of init_noise_sigma generation

* minor fixes

* use partition_point instead of custom bsearch

* Fix some clippy lints.

---------

Co-authored-by: laurent <laurent.mazare@gmail.com>

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

@@ -25,6 +25,22 @@ pub enum PredictionType {
     Sample,
 }
 
+/// Time step spacing for the diffusion process.
+///
+/// "linspace", "leading", "trailing" corresponds to annotation of Table 2. of https://arxiv.org/abs/2305.08891
+#[derive(Debug, Clone, Copy)]
+pub enum TimestepSpacing {
+    Leading,
+    Linspace,
+    Trailing,
+}
+
+impl Default for TimestepSpacing {
+    fn default() -> Self {
+        Self::Leading
+    }
+}
+
 /// Create a beta schedule that discretizes the given alpha_t_bar function, which defines the cumulative product of
 /// `(1-beta)` over time from `t = [0,1]`.
 ///