Bring in improvements from modded-nanogpt repo

[leloykun]

Adds code for:

1. Optimizing Newton-Schulz coefficients
2. Tighter estimate of spectral norm using Gram iteration taken from https://arxiv.org/pdf/2305.16173

---

Usage note:

def zeropower_via_newtonschulz5(
    G: Tensor, steps: int, enable_better_spec_norm_est: bool = False
) -> Tensor:
    assert G.ndim >= 2 # batched Muon implementation by @scottjmaddox, and put into practice in the record by @YouJiacheng
-     a, b, c = (3.4445, -4.7750,  2.0315)
    X = G.bfloat16()
    if G.size(-2) > G.size(-1):
        X = X.mT

    # Ensure spectral norm is at most 1
    X = X / (X.norm(dim=(-2, -1), keepdim=True) + 1e-7)
    # Perform the NS iterations
-     for i in range(steps):
+     for i, (a, b, c) in enumerate([
+         ...[insert the coefficients here]...
+     ]):
        A = X @ X.mT
        if i == 0 and enable_better_spec_norm_est:
            # Tigher estimate of spectral norm using 1st Gram iteration.
            # Taken from https://arxiv.org/pdf/2305.16173
            S_norm_est_over_f_norm__squared = A.norm(dim=(-2, -1), keepdim=True)
            X = X / (S_norm_est_over_f_norm__squared**0.5 + 1e-7)
            A = A / (S_norm_est_over_f_norm__squared + 1e-7)
        B = b * A + c * A @ A # quintic computation strategy adapted from suggestion by @jxbz, @leloykun, and @YouJiacheng
        X = a * X + B @ X
    
    if G.size(-2) > G.size(-1):
        X = X.mT
    return X

[KellerJordan]

have we confirmed that this option never causes any instability?

It's potentially risky, so important to confirm; so I will wait to accept it until I have evidence

[toothacher17]

Hey, @leloykun I tried your Jax scripts and get a group of new hyper coeffs:

(4.0246, -6.4224, 2.6026)
(3.9872, -6.2793, 2.5377)
(3.3260, -4.8258, 1.9451)
(2.8778, -3.6189, 1.6208)
(3.0133, -3.6424, 1.6122)

Do you have any recommendations for which one to use or I just pick a random one?

[leloykun]

Hi @toothacher17,

In zeropower_via_newtonschulz5, you should replace

for i in range(steps):

with

for i, (a, b, c) in enumerate([
    ...[insert the coefficients here]...
])

[KellerJordan]

I'm still a little afraid of this causing instability. Will test more and think about it.

[leloykun]

Same... I'll move this back to drafts until further analysis.