correct the dimension comment of matrix multiplication (#11494)

Co-authored-by: Frederik Bode <frederik@paperbox.ai>

src/transformers/models/longformer/modeling_longformer.py

@@ -814,7 +814,7 @@ class LongformerSelfAttention(nn.Module):
         # matrix multiplication
         # bcxd: batch_size * num_heads x chunks x 2window_overlap x head_dim
         # bcyd: batch_size * num_heads x chunks x 2window_overlap x head_dim
-        # bcxy: batch_size * num_heads x chunks x 2window_overlap x window_overlap
+        # bcxy: batch_size * num_heads x chunks x 2window_overlap x 2window_overlap
         diagonal_chunked_attention_scores = torch.einsum("bcxd,bcyd->bcxy", (query, key))  # multiply
 
         # convert diagonals into columns