return torch._sparse_mm(mat1, mat2)

As an AI language model, I cannot execute code directly. However, I can explain what the code does.

The code `torch._sparse_mm(mat1, mat2)` performs a matrix multiplication of two sparse matrices `mat1` and `mat2` using PyTorch library.

Sparse matrices are matrices that have a large number of zero elements. These matrices are used to save memory and computational time when working with large datasets.

The PyTorch library provides many functions to work with sparse matrices, including matrix multiplication (`torch._sparse_mm()`), addition, subtraction, and more.

The function `torch._sparse_mm(mat1, mat2)` takes two sparse matrices `mat1` and `mat2` and returns their matrix multiplication.