can I leave guangzhou by high speed rail to Hong Kong under the 144 hr visa free policy ?

No, you are not allowed to do that. Anyway, you are perfectly allowed to take a direct through train called Guangzhou-Kowloon through train that will bring you from Guangzhou East station to Hung Hom station in Hong Hong for just 2 hours. In order to obtain 144 hours visa free transit in Guangdong province you HAVE TO buy this ticket (or some other, for example a ferry ticket) in advance and have this ticket (or a confirmation) before you even fly to Guangzhou! In order to buy this ticket you should visit Guangdong Line Fare - MTR Intercity Through Train website.