Add last token pooling support for ORT. (#664)

Co-authored-by: Alvaro Bartolome <36760800+alvarobartt@users.noreply.github.com>

Author: tpendragon

backends/ort/src/lib.rs

@@ -31,7 +31,7 @@ impl OrtBackend {
         let pool = match model_type {
             ModelType::Classifier => Pool::Cls,
             ModelType::Embedding(pool) => match pool {
-                Pool::Splade | Pool::LastToken => {
+                Pool::Splade => {
                     return Err(BackendError::Start(format!(
                         "Pooling {pool} is not supported for this backend. Use `candle` backend instead."
                     )));
@@ -204,8 +204,10 @@ impl Backend for OrtBackend {
             let pooled_embeddings = match self.pool {
                 // CLS pooling
                 Pool::Cls => outputs.slice(s![.., 0, ..]).into_owned().into_dyn(),
-                // Last token pooling is not supported for this model
-                Pool::LastToken => unreachable!(),
+                Pool::LastToken => {
+                    let axis_len = outputs.len_of(Axis(1));
+                    outputs.slice(s![.., axis_len - 1, ..]).into_owned().into_dyn()
+                },
                 // Mean pooling
                 Pool::Mean => {
                     if masking {