Userwarning solve and sample update

[zjZSTU]

first

there are 2 userwarning.

1. The epoch parameter in scheduler.step() was not necessary and is being deprecated where possible. Please use scheduler.step() to step the scheduler.
2. To get the last learning rate computed by the scheduler, please use get_last_lr().

I modified the code to make sure it met the requirements of pytorch

second

use warnup + CosineAnnearling as an sample，and plot the lr list. by the way, i update the readme