1 (尤其当 had 为助动词时，who had 的常用口语形式) Who'd is the usual spoken form of 'who had', especially when 'had' is an auxiliary verb. (尤其当 had 为助动词时，who had 的常用口语形式)

2 （who would 的一种口语形式） Who'd is a spoken form of 'who would'. （who would 的一种口语形式）