bert attention mask (#1934)

* bert attention mask

* Allow for using None as a mask.

* Revert part of the changes so that the proper default mask applies.

* Cosmetic change.

* Another cosmetic tweak.

---------

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

candle-wasm-examples/bert/src/bin/m.rs

@@ -55,11 +55,21 @@ impl Model {
                 Tensor::new(tokens.as_slice(), device)
             })
             .collect::<Result<Vec<_>, _>>()?;
+        let attention_mask: Vec<Tensor> = tokens
+            .iter()
+            .map(|tokens| {
+                let tokens = tokens.get_attention_mask().to_vec();
+                Tensor::new(tokens.as_slice(), device)
+            })
+            .collect::<Result<Vec<_>, _>>()?;
 
         let token_ids = Tensor::stack(&token_ids, 0)?;
+        let attention_mask = Tensor::stack(&attention_mask, 0)?;
         let token_type_ids = token_ids.zeros_like()?;
         console_log!("running inference on batch {:?}", token_ids.shape());
-        let embeddings = self.bert.forward(&token_ids, &token_type_ids)?;
+        let embeddings = self
+            .bert
+            .forward(&token_ids, &token_type_ids, Some(&attention_mask))?;
         console_log!("generated embeddings {:?}", embeddings.shape());
         // Apply some avg-pooling by taking the mean embedding value for all tokens (including padding)
         let (_n_sentence, n_tokens, _hidden_size) = embeddings.dims3()?;