`mamba_split_conv1d_scan_combined` and `mamba_split_conv1d_scan_ref` exhibit large numerical differences when `ngroups > 1`

[HanGuo97]

First of all, thanks for this amazing library!

This is likely not the intended use case of the library, but I noticed (by chance) that the behavior of mamba_split_conv1d_scan_combined seems different than its reference(?) Python implementation mamba_split_conv1d_scan_ref when using ngroups > 1.

import torch
from mamba_ssm.ops.triton.ssd_combined import (
    mamba_split_conv1d_scan_ref,
    mamba_split_conv1d_scan_combined)

def f(ngroups: int) -> None:
    batch_size = 2
    seqlen = 2048
    dim = 1024 * 2
    headdim = 64
    dstate = 128
    nheads = dim // headdim
    conv_dim = 4
    chunk_size = 128
    dtype = torch.float32
    device = "cuda"

    zxbcdt = torch.ones(batch_size, seqlen, 2 * dim + 2 * ngroups * dstate + nheads, device=device, dtype=dtype) / 10.
    conv1d_weight = torch.ones(dim + 2 * ngroups * dstate, conv_dim, device=device, dtype=dtype) / 10.
    conv1d_bias = torch.ones(dim + 2 * ngroups * dstate, device=device, dtype=dtype) / 10.
    dt_bias = torch.ones(nheads, device=device, dtype=dtype) / 10.
    A = (-torch.exp(torch.zeros(nheads, device=device, dtype=dtype)))
    rmsnorm_weight = torch.ones(dim, device=device, dtype=dtype) / 10.
    rmsnorm_eps = 1e-5
    outproj_weight = torch.ones(dim, dim, device=device, dtype=dtype) / 10.
    outproj_bias = torch.ones(dim, device=device, dtype=dtype) / 10.

    # make arguments different by heads (similar effects can be observed by setting other arguments too)
    D = torch.arange(nheads, device=device, dtype=dtype) * 1000.

    out_ref1 = mamba_split_conv1d_scan_combined(
        zxbcdt=zxbcdt,
        conv1d_weight=conv1d_weight,
        conv1d_bias=conv1d_bias,
        dt_bias=dt_bias,
        A=A,
        D=D,
        chunk_size=chunk_size,
        initial_states=None,
        seq_idx=None,
        dt_limit=(0.0, float("inf")),
        return_final_states=False,
        activation="silu",
        rmsnorm_weight=rmsnorm_weight,
        rmsnorm_eps=rmsnorm_eps,
        outproj_weight=outproj_weight,
        outproj_bias=outproj_bias,
        headdim=headdim,
        ngroups=ngroups,
        norm_before_gate=False)

    out_ref2 = mamba_split_conv1d_scan_ref(
        zxbcdt=zxbcdt,
        conv1d_weight=conv1d_weight,
        conv1d_bias=conv1d_bias,
        dt_bias=dt_bias,
        A=A,
        D=D,
        chunk_size=chunk_size,
        # initial_states=None,
        # seq_idx=None,
        dt_limit=(0.0, float("inf")),
        # return_final_states=False,
        activation="silu",
        rmsnorm_weight=rmsnorm_weight,
        rmsnorm_eps=rmsnorm_eps,
        outproj_weight=outproj_weight,
        outproj_bias=outproj_bias,
        headdim=headdim,
        ngroups=ngroups,
        norm_before_gate=False)

    print("mamba_split_conv1d_scan_combined")
    print(out_ref1[0, 0, 0])
    print()
    print("mamba_split_conv1d_scan_ref")
    print(out_ref2[0, 0, 0])

for ngroups in [1, 2, 4]:
    print(f"\n\n------- ngroups = {ngroups} -------")
    f(ngroups=ngroups)

The above code will print the following

------- ngroups = 1 -------
mamba_split_conv1d_scan_combined
tensor(17.6950, device='cuda:0')

mamba_split_conv1d_scan_ref
tensor(17.6951, device='cuda:0')


------- ngroups = 2 -------
mamba_split_conv1d_scan_combined
tensor(18.8725, device='cuda:0')

mamba_split_conv1d_scan_ref
tensor(17.6951, device='cuda:0')


------- ngroups = 4 -------
mamba_split_conv1d_scan_combined
tensor(19.5926, device='cuda:0')

mamba_split_conv1d_scan_ref
tensor(17.6951, device='cuda:0')

Did I miss certain arguments, or is having ngroups > 1 simply a not supported feature? Thanks in advance!