add: pad token

special_tokens.json

@@ -1,4 +1,5 @@
 {
-  "<|endoftext|>": 50256,
-  "<|mdm_mask|>": 50257
+  "<|pad|>": 50256,
+  "<|endoftext|>": 50257,
+  "<|mdm_mask|>": 50258
 }