Pad buffer at the end

ml-agents/mlagents/trainers/buffer.py

                 else:
                     # We want to duplicate the last value in the array, multiplied by the padding_value.
                     padding = np.array(self[-1], dtype=np.float32) * self.padding_value
-                return [padding] * (training_length - leftover) + self[:]
+                return self[:] + [padding] * (training_length - leftover)
 
             else:
                 return self[len(self) - batch_size * training_length :]