interface.cpp

#include <torch/extension.h>
#include <ATen/Generator.h>
#include <ATen/cuda/CUDAGeneratorImpl.h>
#include <vector>

std::vector<c10::optional<torch::Tensor>> fwd_cuda(
    bool is_training,
    const torch::Tensor &input, 
    float dropout_prob,
    c10::optional<at::Generator> gen_
);

torch::Tensor bwd_cuda(
    torch::Tensor &output_grads,
    const torch::Tensor &softmax_results,
    const c10::optional<torch::Tensor> &dropout_mask,
    float dropout_prob
);

// C++ interface

#define CHECK_CUDA(x) AT_ASSERTM(x.is_cuda(), #x " must be a CUDA tensor")
#define CHECK_CONTIGUOUS(x) AT_ASSERTM(x.is_contiguous(), #x " must be contiguous")
#define CHECK_INPUT(x) CHECK_CUDA(x); CHECK_CONTIGUOUS(x)

std::vector<c10::optional<torch::Tensor>> fwd(
    bool is_training,
    const torch::Tensor &input,
    float dropout_prob,
    c10::optional<at::Generator> gen_
) {
    CHECK_INPUT(input);
    AT_ASSERTM(input.dim() == 3, "expected 3D tensor");
    AT_ASSERTM(input.scalar_type() == at::ScalarType::Half ||
               input.scalar_type() == at::ScalarType::BFloat16 ||
               input.scalar_type() == at::ScalarType::Float, "Only HALF/BFloat16/Float is supported");
    return fwd_cuda(is_training, input, dropout_prob, gen_);
}

torch::Tensor bwd(
    torch::Tensor &output_grads, 
    const torch::Tensor &softmax_results,
    const c10::optional<torch::Tensor> &dropout_mask,
    float dropout_prob
) {
    CHECK_INPUT(output_grads);
    CHECK_INPUT(softmax_results);
    if (dropout_mask) {
        CHECK_INPUT(dropout_mask.value());
    }
    AT_ASSERTM(output_grads.dim()    == 3, "expected 3D tensor");
    AT_ASSERTM(softmax_results.dim() == 3, "expected 3D tensor");
    AT_ASSERTM(!dropout_mask || dropout_mask->dim() == 1, "expected 1D tensor");

    AT_ASSERTM(output_grads.scalar_type() == at::ScalarType::Half ||
               output_grads.scalar_type() == at::ScalarType::BFloat16 ||
               output_grads.scalar_type() == at::ScalarType::Float, "Only HALF/BFloat16/Float is supported");
    AT_ASSERTM(softmax_results.scalar_type() == at::ScalarType::Half ||
               softmax_results.scalar_type() == at::ScalarType::BFloat16 ||
               softmax_results.scalar_type() == at::ScalarType::Float, "Only HALF/BFloat16/Float is supported");
    AT_ASSERTM(output_grads.scalar_type() == softmax_results.scalar_type(), "the types mismatch");
    return bwd_cuda(output_grads, softmax_results, dropout_mask, dropout_prob);
}

PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
    m.def("forward", &fwd, "softmax dropout -- Forward.");
    m.def("backward", &bwd, "softmax dropout -- Backward.");
}