Ignore strides CHECK in FromTorch when input tensor is empty

k2/csrc/ragged_ops.cu

@@ -999,7 +999,10 @@ RaggedShape Transpose(RaggedShape &src, Array1<int32_t> *value_indexes) {
   K2_CHECK_GT(src.NumAxes(), 2);
   ContextPtr c = src.Context();
   int32_t src_dim0 = src.Dim0(), src_tot_size1 = src.TotSize(1);
-  if (src_dim0 <= 0) return src;
+  if (src_dim0 <= 0) {
+    if (value_indexes) *value_indexes = Array1<int32_t>(c, 0);
+    return src;
+  }
   int32_t src_dim1 = src_tot_size1 / src_dim0;
   K2_CHECK_EQ(src_tot_size1 % src_dim0, 0)
       << "Transpose(): all dims on axis 0 must be the same.\n"

k2/csrc/ragged_test.cu

@@ -763,6 +763,62 @@ template <typename T>
 void TestTransposeRagged() {
   ContextPtr cpu = GetCpuContext();  // will be used to copy data
   for (auto &context : {GetCpuContext(), GetCudaContext()}) {
+    // empty case, fsavec with a empty fsa
+    {
+      const std::vector<int32_t> row_splits1_vec = {0, 0};
+      const std::vector<int32_t> row_splits2_vec = {0};
+      Array1<int32_t> row_splits1(context, row_splits1_vec);
+      Array1<int32_t> row_splits2(context, row_splits2_vec);
+      RaggedShape src_shape =
+          RaggedShape3(&row_splits1, nullptr, -1, &row_splits2, nullptr, -1);
+      ASSERT_EQ(src_shape.Dim0(), 1);
+      ASSERT_EQ(src_shape.TotSize(1), 0);
+
+      Array1<T> values_array(context, 0);
+      ASSERT_EQ(values_array.Dim(), src_shape.NumElements());
+
+      Ragged<T> ragged(src_shape, values_array);
+      Ragged<T> ans = Transpose(ragged);
+      RaggedShape shape = ans.shape;
+      // Check shape
+      ASSERT_EQ(shape.Dim0(), 0);
+      ASSERT_EQ(shape.TotSize(1), 0);
+      CheckArrayData(shape.RowSplits(1), std::vector<int32_t>({0}));
+      CheckArrayData(shape.RowSplits(2), std::vector<int32_t>({0}));
+      K2_CHECK_EQ(shape.RowIds(1).Dim(), 0);
+      K2_CHECK_EQ(shape.RowIds(2).Dim(), 0);
+      // Check values
+      K2_CHECK_EQ(ans.values.Dim(), 0);
+    }
+
+    // empty case, fsavec without any fsa
+    {
+      const std::vector<int32_t> row_splits1_vec = {0};
+      const std::vector<int32_t> row_splits2_vec = {0};
+      Array1<int32_t> row_splits1(context, row_splits1_vec);
+      Array1<int32_t> row_splits2(context, row_splits2_vec);
+      RaggedShape src_shape =
+          RaggedShape3(&row_splits1, nullptr, -1, &row_splits2, nullptr, -1);
+      ASSERT_EQ(src_shape.Dim0(), 0);
+      ASSERT_EQ(src_shape.TotSize(1), 0);
+
+      Array1<T> values_array(context, 0);
+      ASSERT_EQ(values_array.Dim(), src_shape.NumElements());
+
+      Ragged<T> ragged(src_shape, values_array);
+      Ragged<T> ans = Transpose(ragged);
+      RaggedShape shape = ans.shape;
+      // Check shape
+      ASSERT_EQ(shape.Dim0(), 0);
+      ASSERT_EQ(shape.TotSize(1), 0);
+      CheckArrayData(shape.RowSplits(1), std::vector<int32_t>({0}));
+      CheckArrayData(shape.RowSplits(2), std::vector<int32_t>({0}));
+      K2_CHECK_EQ(shape.RowIds(1).Dim(), 0);
+      K2_CHECK_EQ(shape.RowIds(2).Dim(), 0);
+      // Check values
+      K2_CHECK_EQ(ans.values.Dim(), 0);
+    }
+
     {
       const std::vector<int32_t> row_splits1_vec = {0, 2, 4, 6};
       const std::vector<int32_t> row_splits2_vec = {0, 3, 4, 7, 8, 10, 12};

k2/python/csrc/torch/torch_util.h

@@ -119,8 +119,11 @@ Array1<T> FromTorch(torch::Tensor &tensor) {
   K2_CHECK_EQ(tensor.scalar_type(), ToScalarType<T>::value)
       << "Expected scalar type: " << ToScalarType<T>::value
       << ". Given: " << tensor.scalar_type();
-  K2_CHECK_EQ(tensor.strides()[0], 1)
-      << "Expected stride: 1. Given: " << tensor.strides()[0];
+  // Some empty tensor may have stride not equal to 1, e.g., tensor returned by
+  // clone() method, it is valid here, so we won't check its strieds.
+  if (tensor.numel())
+    K2_CHECK_EQ(tensor.strides()[0], 1)
+        << "Expected stride: 1. Given: " << tensor.strides()[0];
 
   auto region = NewRegion(tensor);
   Array1<T> ans(tensor.numel(), region, 0);