Creat random key automatically when it is detected

[chaoming0625]

Every time when calling brainpy.math.clear_buffer_memory(), the random function in brainpy.math.random will not be available except when recalling brainpy.math.random.seed().

This PR fixes this problem and every time after the clear_buffer_memory is called, the random key of brainpy.math.random.DEFAULT will be regenerated automatically.