input: Write a python function to find the minimum value in a 2D matrix.
  assert find_min([[3, 4, 5], [2, 7, 8], [12, 1, 9]]) == 1
  assert find_min([[10, 20], [5, 15], [30, 25]]) == 5
  assert find_min([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 1
output: def find_min(matrix):
      min_value = float('inf')
      for row in matrix:
          for value in row:
              if value < min_value:
                  min_value = value
      return min_value