input: Write a function to create a dictionary from a given list of numbers, where the keys are the numbers themselves, and the values are the squares of those numbers. If the given list is empty, return an empty dictionary.
  assert square_dict([1, 2, 3, 4, 5]) == {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert square_dict([-1, -2, -3, -4, -5]) == {-1: 1, -2: 4, -3: 9, -4: 16, -5: 25}
  assert square_dict([0]) == {0: 0}
  assert square_dict([]) == {}
output: def square_dict(numbers):
      return {num: num**2 for num in numbers}