You write custom CUDA kernels to replace the pytorch operators in the given architecture to get speedups. 

    You have complete freedom to choose the set of operators you want to replace. You may make the decision to replace some operators with custom CUDA kernels and leave others unchanged. You may replace multiple operators with custom implementations, consider operator fusion opportunities (combining multiple operators into a single kernel, for example, combining matmul+relu), or algorithmic changes (such as online softmax). You are only limited by your imagination.


        Here's an example to show you the syntax of inline embedding custom CUDA operators in torch: The example given architecture is: 

        ``` 

        # Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the license found in the
# LICENSE file in the root directory of this source tree.

import torch
import torch.nn as nn
import torch.nn.functional as F


class Model(nn.Module):
    def __init__(self) -> None:
        super().__init__()

    def forward(self, a, b):
        return a + b


def get_inputs():
    # randomly generate input tensors based on the model architecture
    a = torch.randn(1, 128).cuda()
    b = torch.randn(1, 128).cuda()
    return [a, b]


def get_init_inputs():
    # randomly generate tensors required for initialization based on the model architecture
    return []

        ``` 

        The example new arch with custom CUDA kernels looks like this:
        ```
        # Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the license found in the
# LICENSE file in the root directory of this source tree.

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.utils.cpp_extension import load_inline

# Define the custom CUDA kernel for element-wise addition
elementwise_add_source = """
#include <torch/extension.h>
#include <cuda_runtime.h>

__global__ void elementwise_add_kernel(const float* a, const float* b, float* out, int size) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < size) {
        out[idx] = a[idx] + b[idx];
    }
}

torch::Tensor elementwise_add_cuda(torch::Tensor a, torch::Tensor b) {
    auto size = a.numel();
    auto out = torch::zeros_like(a);

    const int block_size = 256;
    const int num_blocks = (size + block_size - 1) / block_size;

    elementwise_add_kernel<<<num_blocks, block_size>>>(a.data_ptr<float>(), b.data_ptr<float>(), out.data_ptr<float>(), size);

    return out;
}
"""

elementwise_add_cpp_source = (
    "torch::Tensor elementwise_add_cuda(torch::Tensor a, torch::Tensor b);"
)

# Compile the inline CUDA code for element-wise addition
elementwise_add = load_inline(
    name="elementwise_add",
    cpp_sources=elementwise_add_cpp_source,
    cuda_sources=elementwise_add_source,
    functions=["elementwise_add_cuda"],
    verbose=True,
    extra_cflags=[""],
    extra_ldflags=[""],
)


class ModelNew(nn.Module):
    def __init__(self) -> None:
        super().__init__()
        self.elementwise_add = elementwise_add

    def forward(self, a, b):
        return self.elementwise_add.elementwise_add_cuda(a, b)

        ``` 

        
    You are given the following architecture: 

    ```
    # Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the license found in the
# LICENSE file in the root directory of this source tree.

import torch
import torch.nn as nn

class Model(nn.Module):
    """
    Simple model that performs matrix-vector multiplication (C = A * B).
    """
    def __init__(self):
        super(Model, self).__init__()
    
    def forward(self, A: torch.Tensor, B: torch.Tensor) -> torch.Tensor:
        """
        Performs matrix-vector multiplication.

        Args:
            A: Input matrix of shape (M, K).
            B: Input vector of shape (K, 1).

        Returns:
            Output vector of shape (M, 1).
        """
        return torch.matmul(A, B)

M = 256 * 8 # 2048
K = 131072 * 8 # 1048576

def get_inputs():
    A = torch.rand(M, K)
    B = torch.rand(K, 1)
    return [A, B]

def get_init_inputs():
    return []  # No special initialization inputs needed
    ```
    
Optimize the architecture named Model with custom CUDA operators! Name your optimized output architecture ModelNew. Output the new code in codeblocks. Please generate real code, NOT pseudocode, make sure the code compiles and is fully functional. Just output the new model code, no other text, and NO testing code! 



This is turn 3 of 6 for this task. Your previous solution (turn 2) was:
```python
# Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the license found in the
# LICENSE file in the root directory of this source tree.

import torch
import torch.nn as nn
from torch.utils.cpp_extension import load_inline

# Fused split-K matrix-vector multiplication (C = A @ B) CUDA kernel.
#
# Workload: A (M, K) = (2048, 1048576), B (K, 1) -> C (M, 1).
# Purely DRAM-bandwidth bound (~8 GiB of A streamed exactly once).
#
# Improvements over a one-block-per-few-rows design:
#   * Split-K: the K dimension is partitioned across kSplitK blocks per row
#     group, giving 4096 independent blocks. This keeps every SM saturated
#     even on high-memory-bandwidth parts and smooths load balance.
#   * float4 (128-bit) fully-coalesced loads.
#   * __ldcs (evict-first streaming) for A: each element is read once, so it
#     must not displace B in L2.
#   * __ldg (read-only) for B: the 4 MiB vector stays resident in L2 and is
#     shared by all blocks; each load feeds kRowsPerBlock dot products.
#   * Loop unrolling for deep memory-level parallelism.
#   * Warp-shuffle + shared-memory block reduction, finished with a single
#     red.global.add (atomicAdd) per row slice - no global synchronization
#     and no second reduction kernel.

matvec_cuda_source = r"""
#include <torch/extension.h>
#include <ATen/cuda/CUDAContext.h>
#include <c10/cuda/CUDAException.h>
#include <cuda_runtime.h>

namespace {

constexpr int kBlockSize    = 256;
constexpr int kNumWarps     = kBlockSize / 32;
constexpr int kRowsPerBlock = 4;
constexpr int kSplitK       = 8;

template <int R>
__global__ void __launch_bounds__(kBlockSize)
matvec_splitk_kernel(const float4* __restrict__ A4,
                     const float4* __restrict__ B4,
                     float* __restrict__ C,
                     const int M,
                     const int K4,
                     const int slice4) {
    const int row0 = blockIdx.x * R;
    const int i0   = blockIdx.y * slice4;
    const int i1   = min(K4, i0 + slice4);

    // One base pointer per row handled by this block (clamped on tail blocks;
    // those recomputed rows contribute a harmless 0.0f and are never the
    // final write target).
    const float4* a_rows[R];
    #pragma unroll
    for (int r = 0; r < R; ++r) {
        int row = row0 + r;
        if (row >= M) row = M - 1;
        a_rows[r] = A4 + static_cast<size_t>(row) * K4;
    }

    float4 acc[R];
    #pragma unroll
    for (int r = 0; r < R; ++r) acc[r] = make_float4(0.f, 0.f, 0.f, 0.f);

    const int stride = kBlockSize;
    int i = i0 + threadIdx.x;

    // Main loop: two float4 column chunks per iteration for extra MLP.
    for (; i + stride < i1; i += 2 * stride) {
        const float4 b0 = __ldg(B4 + i);
        const float4 b1 = __ldg(B4 + i + stride);
        #pragma unroll
        for (int r = 0; r < R; ++r) {
            const float4 a0 = __ldcs(a_rows[r] + i);
            const float4 a1 = __ldcs(a_rows[r] + i + stride);
            acc[r].x += a0.x * b0.x + a1.x * b1.x;
            acc[r].y += a0.y * b0.y + a1.y * b1.y;
            acc[r].z += a0.z * b0.z + a1.z * b1.z;
            acc[r].w += a0.w * b0.w + a1.w * b1.w;
        }
    }
    // Slice remainder.
    for (; i < i1; i += stride) {
        const float4 b = __ldg(B4 + i);
        #pragma unroll
        for (int r = 0; r < R; ++r) {
            const float4 a = __ldcs(a_rows[r] + i);
            acc[r].x += a.x * b.x;
            acc[r].y += a.y * b.y;
            acc[r].z += a.z * b.z;
            acc[r].w += a.w * b.w;
        }
    }

    // Collapse float4 lanes, then reduce across the block.
    float sum[R];
    #pragma unroll
    for (int r = 0; r < R; ++r) {
        sum[r] = (acc[r].x + acc[r].y) + (acc[r].z + acc[r].w);
    }

    #pragma unroll
    for (int r = 0; r < R; ++r) {
        #pragma unroll
        for (int off = 16; off > 0; off >>= 1) {
            sum[r] += __shfl_down_sync(0xffffffffu, sum[r], off);
        }
    }

    __shared__ float warp_sums[R][kNumWarps];
    const int lane = threadIdx.x & 31;
    const int wid  = threadIdx.x >> 5;

    if (lane == 0) {
        #pragma unroll
        for (int r = 0; r < R; ++r) warp_sums[r][wid] = sum[r];
    }
    __syncthreads();

    if (wid == 0) {
        #pragma unroll
        for (int r = 0; r < R; ++r) {
            float s = (lane < kNumWarps) ? warp_sums[r][lane] : 0.f;
            #pragma unroll
            for (int off = kNumWarps / 2; off > 0; off >>= 1) {
                s += __shfl_down_sync(0xffffffffu, s, off);
            }
            if (lane == 0) {
                const int row = row0 + r;
                if (row < M) atomicAdd(&C[row], s);
            }
        }
    }
}

}  // namespace

torch::Tensor matvec_cuda(torch::Tensor A, torch::Tensor B) {
    TORCH_CHECK(A.is_cuda() && B.is_cuda(), "A and B must be CUDA tensors");
    TORCH_CHECK(A.scalar_type() == at::kFloat && B.scalar_type() == at::kFloat,
                "A and B must be float32");
    TORCH_CHECK(A.dim() == 2 && B.dim() == 2 && B.size(1) == 1,
                "Expected shapes A (M, K) and B (K, 1)");
    TORCH_CHECK(A.size(1) == B.size(0), "Inner dimensions must match");

    const int64_t M = A.size(0);
    const int64_t K = A.size(1);
    auto C = torch::empty({M, 1}, A.options());

    if (M == 0 || K == 0) {
        return C.zero_();
    }
    if ((K & 3) != 0) {
        // Alignment fallback (never triggered by this model's shapes).
        C.copy_(torch::matmul(A, B));
        return C;
    }

    const int K4 = static_cast<int>(K >> 2);

    // Split the K dimension so that roughly kSplitK blocks cooperate on each
    // row group. Keep slices a multiple of the block size so every warp's
    // accesses stay 128-byte aligned.
    int slice4 = (K4 + kSplitK - 1) / kSplitK;
    slice4 = ((slice4 + kBlockSize - 1) / kBlockSize) * kBlockSize;
    const int num_slices = (K4 + slice4 - 1) / slice4;

    const dim3 grid(
        static_cast<unsigned int>((M + kRowsPerBlock - 1) / kRowsPerBlock),
        static_cast<unsigned int>(num_slices));

    cudaStream_t stream = at::cuda::getCurrentCUDAStream();
    C10_CUDA_CHECK(cudaMemsetAsync(C.data_ptr<float>(), 0,
                                   static_cast<size_t>(M) * sizeof(float),
                                   stream));

    matvec_splitk_kernel<kRowsPerBlock><<<grid, kBlockSize, 0, stream>>>(
        reinterpret_cast<const float4*>(A.data_ptr<float>()),
        reinterpret_cast<const float4*>(B.data_ptr<float>()),
        C.data_ptr<float>(),
        static_cast<int>(M),
        K4,
        slice4);
    C10_CUDA_KERNEL_LAUNCH_CHECK();

    return C;
}
"""

matvec_cpp_source = "torch::Tensor matvec_cuda(torch::Tensor A, torch::Tensor B);"

matvec_ext = load_inline(
    name="matvec_splitk_ext",
    cpp_sources=matvec_cpp_source,
    cuda_sources=matvec_cuda_source,
    functions=["matvec_cuda"],
    verbose=False,
    extra_cflags=["-O3"],
    extra_cuda_cflags=["-O3"],
)


class ModelNew(nn.Module):
    """
    Optimized matrix-vector multiplication (C = A @ B) backed by a custom
    split-K, bandwidth-saturating CUDA kernel. Falls back to torch.matmul
    for any unsupported input configuration.
    """

    def __init__(self):
        super(ModelNew, self).__init__()
        self._fn = matvec_ext.matvec_cuda

    def forward(self, A: torch.Tensor, B: torch.Tensor) -> torch.Tensor:
        if (
            A.is_cuda
            and B.is_cuda
            and A.dtype == torch.float32
            and B.dtype == torch.float32
            and A.dim() == 2
            and B.dim() == 2
            and B.size(1) == 1
            and A.size(1) == B.size(0)
        ):
            return self._fn(A.contiguous(), B.contiguous())
        return torch.matmul(A, B)
```
Result of evaluating it:
The solution is correct on the test inputs. Mean runtime 5.78 ms; the PyTorch reference (with TF32 enabled) takes 6.1 ms; speedup 1.055x.
Profiler, top device kernels of one forward pass of your solution:
| device kernel | total device time (us) | calls |
|---|---|---|
| void (anonymous namespace)::matvec_splitk_kernel<4>(float4 const*, float4 const*, float*,  | 5794.5 | 1 |
| Memset (Device) | 1.8 | 1 |

Write an improved complete ModelNew: fix any error, and if the solution is already correct, make it faster. Output the new code in one ```python codeblock. Just the model code, no other text, and NO testing code.
