chore: import upstream snapshot with attribution

dgl_sparse/include/sparse/softmax.h

@@ -0,0 +1,33 @@
+/**
+ *  Copyright (c) 2022 by Contributors
+ * @file sparse/softmax.h
+ * @brief DGL C++ Softmax operator
+ */
+#ifndef SPARSE_SOFTMAX_H_
+#define SPARSE_SOFTMAX_H_
+
+#include <sparse/sparse_matrix.h>
+
+namespace dgl {
+namespace sparse {
+
+/**
+ * @brief Apply softmax to the non-zero entries of the sparse matrix on the
+ * dimension dim. dim = 0 or 1 indicates column-wise or row-wise softmax
+ * respectively.
+ *
+ * This function supports autograd for the sparse matrix, but it does not
+ * support higher order gradient.
+ *
+ * @param sparse_mat The sparse matrix
+ * @param dim The dimension to apply softmax
+ *
+ * @return Sparse matrix
+ */
+c10::intrusive_ptr<SparseMatrix> Softmax(
+    const c10::intrusive_ptr<SparseMatrix>& sparse_mat, int64_t dim);
+
+}  // namespace sparse
+}  // namespace dgl
+
+#endif  // SPARSE_SOFTMAX_H_