input: Write a python function to pad a matrix with zeros on the right and bottom to match the given pad size.
  assert pad_matrix([[1, 2], [3, 4]], 4) == [[1, 2, 0, 0], [3, 4, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
  assert pad_matrix([[5, 6], [7, 8]], 3) == [[5, 6, 0], [7, 8, 0], [0, 0, 0]]
  assert pad_matrix([[1]], 2) == [[1, 0], [0, 0]]
output: def pad_matrix(matrix, pad_size):
      rows = len(matrix)
      cols = len(matrix[0]) if matrix else 0
  
      padded_matrix = []
      for i in range(pad_size):
          row = []
          for j in range(pad_size):
              if i < rows and j < cols:
                  row.append(matrix[i][j])
              else:
                  row.append(0)
          padded_matrix.append(row)
  
      return padded_matrix