input: Write a python function to check if a matrix is symmetric.
  assert is_symmetric([[1, 2, 3], [2, 0, 4], [3, 4, 5]]) == True
  assert is_symmetric([[1, 2, 3], [-2, 0, 4], [3, -4, 5]]) == False
  assert is_symmetric([[1, 0, 0], [0, 1, 0], [0, 0, 1]]) == True
output: def is_symmetric(matrix):
      n = len(matrix)
      for i in range(n):
          for j in range(n):
              if matrix[i][j] != matrix[j][i]:
                  return False
      return True