Add implementation of Lion optimiser

[mashu]

Implementation of Lion optimiser which is faster than AdamW from Symbolic Discovery of Optimization Algorithms

[mashu]

@chengchingwen I amended the commit so it should now reflect your suggestions.

[mcabbott]

For easier reference the paper's description (including weight decay) is:

Later, in the appendix, they have pseudo-code without weight decay:

Note that this needs m_{t-1} but never c_{t-1}, which is why they advertise it as needing to store fewer arrays than Adam, i.e. like one not two copies of the parameters:

[chengchingwen]

Looks like it's ready to go?