Add EMA with nesterov - Muon.

[enriquezaf]

Current version is kinda exploding the gradients, non standard ema, looks to me is half way into using 2 different betas but never got done.

Current

• step 1: buf * momentum
• step 2: grad + buf * momentum
• step 3: grad + buf * momentum * (grad + buf * momentum)

buf.mul_(momentum)       # buf * momentum
buf.add_(grad)           # grad + buf * momentum
grad.add_(buf*momentum)  # grad + buf * momentum * (grad + buf * momentum)

# if momentum = 0.9
# 1: buf * 0.9
# 2: grad + 0.9 * buf
# 3: grad + 0.9 * (grad + 0.9 * buf) -> grad + 0.9 * grad + 0.81 * buf
# -> grad = 1.9 * grad + 0.81 * buf

New

# if momentum = 0.9
# grad = buf * 0.9 + 0.1 * grad

This match the default from heavyball.