Fix indexing of lasttoken pooling for longest sequence (#2111)

* Update Pooling.py

fixing last token indexing for sequences that span the entire length

* Update Pooling.py

change to seq_len

* Typo: lenth -> length

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Co-authored-by: Tom Aarsen <Cubiegamedev@gmail.com>

sentence_transformers/models/Pooling.py

@@ -139,7 +139,9 @@ def forward(self, features: Dict[str, Tensor]):
             # attention_mask shape: (bs, seq_len)
             # Get shape [bs] indices of the last token (i.e. the last token for each batch item)
             # argmin gives us the index of the first 0 in the attention mask; We get the last 1 index by subtracting 1
-            gather_indices = torch.argmin(attention_mask, 1, keepdim=False) - 1 # Shape [bs]
+            # Any sequence where min == 1, we use the entire sequence length since argmin = 0
+            values, indices = torch.min(attention_mask, 1, keepdim = False)
+            gather_indices = torch.where(values==0, indices, seq_len) - 1 # Shape [bs]
 
             # There are empty sequences, where the index would become -1 which will crash
             gather_indices = torch.clamp(gather_indices, min=0)