fix bug with padding mask + add corresponding test

examples/utils_summarization.py

@@ -127,9 +127,9 @@ def build_lm_labels(sequence, pad_token):
 def build_mask(sequence, pad_token):
     """ Builds the mask. The attention mechanism will only attend to positions
     with value 1. """
-    mask = sequence.clone()
-    mask[mask != pad_token] = 1
-    mask[mask == pad_token] = 0
+    mask = torch.ones_like(sequence)
+    idx_pad_tokens = (sequence == pad_token)
+    mask[idx_pad_tokens] = 0
     return mask