```python
import weakref

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


_cpp_source = r"""
#include <torch/extension.h>

torch::Tensor pack_weight_cuda(torch::Tensor weight);

torch::Tensor gemm_scale_leaky_cuda(
    torch::Tensor x,
    torch::Tensor packed_weight,
    torch::Tensor bias,
    double multiplier,
    double negative_slope);
"""

_cuda_source = r"""
#include <torch/extension.h>
#include <ATen/cuda/CUDAContext.h>
#include <c10/cuda/CUDAGuard.h>
#include <c10/cuda/CUDAException.h>
#include <cuda_fp16.h>
#include <cuda_runtime.h>
#include <cublas_v2.h>
#include <climits>
#include <cstdint>

template<int Scale>
__global__ void pack_scaled(
    const float4* __restrict__ input,
    __half2* __restrict__ output,
    int64_t vectors) {
    const int64_t i =
        static_cast<int64_t>(blockIdx.x) * blockDim.x + threadIdx.x;
    if (i >= vectors) return;

    const float4 v = input[i];
    output[2 * i] = __floats2half2_rn(
        v.x * float(Scale), v.y * float(Scale));
    output[2 * i + 1] = __floats2half2_rn(
        v.z * float(Scale), v.w * float(Scale));
}

__device__ __forceinline__ float activate(
    float value, float bias, float multiplier, float slope) {
    const float y = __fmul_rn(__fadd_rn(value, bias), multiplier);
    return y > 0.0f ? y : __fmul_rn(y, slope);
}

__global__ void epilogue_vec4(
    float4* __restrict__ output,
    const float4* __restrict__ bias,
    int64_t vectors,
    int channels4,
    float multiplier,
    float slope) {
    const int64_t i =
        static_cast<int64_t>(blockIdx.x) * blockDim.x + threadIdx.x;
    if (i >= vectors) return;

    float4 v = output[i];
    const float4 b = bias[i % channels4];
    v.x = activate(v.x, b.x, multiplier, slope);
    v.y = activate(v.y, b.y, multiplier, slope);
    v.z = activate(v.z, b.z, multiplier, slope);
    v.w = activate(v.w, b.w, multiplier, slope);
    output[i] = v;
}

torch::Tensor pack_weight_cuda(torch::Tensor weight) {
    TORCH_CHECK(
        weight.is_cuda() && weight.scalar_type() == at::kFloat &&
        weight.dim() == 2 && weight.is_contiguous(),
        "Expected contiguous CUDA float32 weight");
    TORCH_CHECK(
        weight.size(0) > 0 && weight.size(1) > 0 &&
        weight.size(0) % 8 == 0 && weight.size(1) % 8 == 0,
        "Weight dimensions must be positive multiples of eight");
    TORCH_CHECK(
        reinterpret_cast<uintptr_t>(weight.data_ptr<float>()) % 16 == 0,
        "Weight must be 16-byte aligned");

    const c10::cuda::CUDAGuard guard(weight.device());
    auto packed = torch::empty(
        weight.sizes(), weight.options().dtype(at::kHalf));
    const int64_t vectors = weight.numel() / 4;
    const auto stream = at::cuda::getCurrentCUDAStream();

    pack_scaled<128><<<
        static_cast<unsigned int>((vectors + 255) / 256),
        256, 0, stream.stream()>>>(
            reinterpret_cast<const float4*>(weight.data_ptr<float>()),
            reinterpret_cast<__half2*>(packed.data_ptr<at::Half>()),
            vectors);
    C10_CUDA_KERNEL_LAUNCH_CHECK();
    return packed;
}

torch::Tensor gemm_scale_leaky_cuda(
    torch::Tensor x,
    torch::Tensor packed_weight,
    torch::Tensor bias,
    double multiplier,
    double negative_slope) {
    TORCH_CHECK(
        x.is_cuda() && packed_weight.is_cuda() && bias.is_cuda(),
        "Expected CUDA tensors");
    TORCH_CHECK(
        x.device() == packed_weight.device() && x.device() == bias.device(),
        "Expected tensors on the same device");
    TORCH_CHECK(
        x.scalar_type() == at::kFloat &&
        packed_weight.scalar_type() == at::kHalf &&
        bias.scalar_type() == at::kFloat,
        "Invalid tensor dtypes");
    TORCH_CHECK(
        x.is_contiguous() && packed_weight.is_contiguous() &&
        bias.is_contiguous(), "Expected contiguous tensors");
    TORCH_CHECK(
        x.dim() >= 1 && packed_weight.dim() == 2 && bias.dim() == 1,
        "Invalid tensor dimensions");

    const int64_t k64 = packed_weight.size(1);
    const int64_t n64 = packed_weight.size(0);
    TORCH_CHECK(
        k64 > 0 && n64 > 0 && k64 % 8 == 0 && n64 % 8 == 0 &&
        x.size(-1) == k64 && bias.numel() == n64,
        "Invalid feature dimensions");

    const int64_t m64 = x.numel() / k64;
    TORCH_CHECK(
        m64 <= INT_MAX && n64 <= INT_MAX && k64 <= INT_MAX,
        "GEMM dimensions exceed supported limits");
    TORCH_CHECK(
        reinterpret_cast<uintptr_t>(x.data_ptr<float>()) % 16 == 0 &&
        reinterpret_cast<uintptr_t>(bias.data_ptr<float>()) % 16 == 0,
        "Input and bias must be 16-byte aligned");

    const c10::cuda::CUDAGuard guard(x.device());
    auto output_sizes = x.sizes().vec();
    output_sizes.back() = n64;
    auto output = torch::empty(output_sizes, x.options());
    if (output.numel() == 0) return output;

    auto packed_x = torch::empty(x.sizes(), x.options().dtype(at::kHalf));
    const auto stream = at::cuda::getCurrentCUDAStream();
    const int64_t input_vectors = x.numel() / 4;

    pack_scaled<16><<<
        static_cast<unsigned int>((input_vectors + 255) / 256),
        256, 0, stream.stream()>>>(
            reinterpret_cast<const float4*>(x.data_ptr<float>()),
            reinterpret_cast<__half2*>(packed_x.data_ptr<at::Half>()),
            input_vectors);
    C10_CUDA_KERNEL_LAUNCH_CHECK();

    const float alpha = 1.0f / 2048.0f;
    const float beta = 0.0f;
    auto handle = at::cuda::getCurrentCUDABlasHandle();

    const cublasStatus_t status = cublasGemmEx(
        handle,
        CUBLAS_OP_T, CUBLAS_OP_N,
        static_cast<int>(n64),
        static_cast<int>(m64),
        static_cast<int>(k64),
        &alpha,
        packed_weight.data_ptr<at::Half>(), CUDA_R_16F,
        static_cast<int>(k64),
        packed_x.data_ptr<at::Half>(), CUDA_R_16F,
        static_cast<int>(k64),
        &beta,
        output.data_ptr<float>(), CUDA_R_32F,
        static_cast<int>(n64),
        CUBLAS_COMPUTE_32F,
        CUBLAS_GEMM_DEFAULT_TENSOR_OP);
    TORCH_CHECK(
        status == CUBLAS_STATUS_SUCCESS,
        "cublasGemmEx failed: ", static_cast<int>(status));

    const int64_t output_vectors = output.numel() / 4;
    epilogue_vec4<<<
        static_cast<unsigned int>((output_vectors + 255) / 256),
        256, 0, stream.stream()>>>(
            reinterpret_cast<float4*>(output.data_ptr<float>()),
            reinterpret_cast<const float4*>(bias.data_ptr<float>()),
            output_vectors,
            static_cast<int>(n64 / 4),
            static_cast<float>(multiplier),
            static_cast<float>(negative_slope));
    C10_CUDA_KERNEL_LAUNCH_CHECK();
    return output;
}
"""

_ops = load_inline(
    name="versioned_packed_gemm_scale_leaky_v5",
    cpp_sources=_cpp_source,
    cuda_sources=_cuda_source,
    functions=["pack_weight_cuda", "gemm_scale_leaky_cuda"],
    extra_cflags=["-O3"],
    extra_cuda_cflags=["-O3"],
    extra_ldflags=["-lcublas"],
    verbose=False,
)


class ModelNew(nn.Module):
    def __init__(self, in_features, out_features, multiplier, negative_slope):
        super().__init__()
        self.gemm = nn.Linear(in_features, out_features)
        self.multiplier = multiplier
        self.leaky_relu = nn.LeakyReLU(negative_slope)
        self.register_buffer("_packed_weight", None, persistent=False)
        self._weight_ref = None
        self._weight_key = None
        self._packing_event = None
        self._packing_stream = None

    def _get_packed_weight(self, weight, stream):
        try:
            version = weight._version
        except RuntimeError:
            return _ops.pack_weight_cuda(weight)

        key = (
            version,
            weight.data_ptr(),
            weight.device,
            tuple(weight.shape),
            tuple(weight.stride()),
        )
        if (
            self._packed_weight is None
            or self._packed_weight.device != weight.device
            or self._packed_weight.dtype != torch.float16
            or self._weight_ref is None
            or self._weight_ref() is not weight
            or self._weight_key != key
        ):
            packed = _ops.pack_weight_cuda(weight)
            event = torch.cuda.Event(enable_timing=False)
            event.record(stream)

            self._packed_weight = packed
            self._weight_ref = weakref.ref(weight)
            self._weight_key = key
            self._packing_event = event
            self._packing_stream = stream.cuda_stream

        if stream.cuda_stream != self._packing_stream:
            stream.wait_event(self._packing_event)
            self._packed_weight.record_stream(stream)

        return self._packed_weight

    def forward(self, x):
        weight = self.gemm.weight
        bias = self.gemm.bias

        if (
            x.is_cuda
            and weight.device == x.device
            and bias is not None
            and bias.device == x.device
            and x.dtype == torch.float32
            and weight.dtype == torch.float32
            and bias.dtype == torch.float32
            and not torch.is_autocast_enabled()
            and x.ndim >= 1
            and weight.shape[1] > 0
            and weight.shape[0] > 0
            and weight.shape[1] % 8 == 0
            and weight.shape[0] % 8 == 0
            and x.shape[-1] == weight.shape[1]
            and weight.is_contiguous()
            and weight.data_ptr() % 16 == 0
        ):
            x = x.contiguous()
            bias = bias.contiguous()
            if x.data_ptr() % 16 == 0 and bias.data_ptr() % 16 == 0:
                stream = torch.cuda.current_stream(x.device)
                packed_weight = self._get_packed_weight(weight, stream)
                return _ops.gemm_scale_leaky_cuda(
                    x,
                    packed_weight,
                    bias,
                    float(self.multiplier),
                    float(self.leaky_relu.negative_slope),
                )

        return self.leaky_relu(self.gemm(x) * self.multiplier)
```