Syntax error in diffgrad.py.

[Phylake1337]

Hello Mr.Shivram.
Firstly, I want to thank you for the great work.
I was trying to use the optimizer in diffgrad.py, but there's a syntax error line 100.
state['previous_grad'] = grad %used in paper but has the bug that previous grad is overwritten with grad and diff becomes always zero. Fixed in the next line.
it should be.
state['previous_grad'] = grad #used in the paper but has the bug that the previous grad is overwritten with grad and diff becomes always zero. Fixed in the next line.
it seems you are coding in c++ much more than python :D.