Fix Split index bugs uncovered by QNN SDK 2.19 (#19381)

### Description
- When converting ONNX split sizes to QNN split indices, do not include
the split at index 0. QNN 2.19 assumes index 0 is implicit and throws a
validation error if provided.
- Fix bug when using an ONNX Split operator with a `num_outputs`
attribute that does not evenly divide into `shape[axis]`. The ONNX spec
states that the last chunk should be smaller, but QNN EP made the last
chunk larger.
- Fix bug when using an ONNX Split operator with a `split` input. QNN EP
was incorrectly passing the split sizes as split indices without
conversion.

### Motivation and Context
QNN SDK 2.19 updated validation criteria for Split operators. QNN EP was
previously passing a split index that should have been implicit.

Also, discovered a bugs when using `num_outputs` attribute and `split`
input.

onnxruntime/core/providers/qnn/builder/opbuilder/split_op_builder.cc

@@ -55,6 +55,19 @@ Status SplitOpBuilder::ProcessInputs(QnnModelWrapper& qnn_model_wrapper,
   return Status::OK();
 }
 
+// Converts an ONNX list of split lengths to a QNN list of split indices.
+// Note that the first split index at 0 is implicit (QNN SDK >= 2.19 will raise a validation error if included).
+static void ConvertSplitLengthsToSplitIndices(gsl::span<const int64_t> split_lengths,
+                                              std::vector<uint32_t>& split_indices) {
+  uint32_t split_it = 0;
+  for (size_t i = 0; i < split_lengths.size(); ++i) {
+    if (i > 0) {  // Do not include the 0th split index.
+      split_indices.push_back(split_it);
+    }
+    split_it += SafeInt<uint32_t>(split_lengths[i]);
+  }
+}
+
 Status SplitOpBuilder::ProcessAttributesAndOutputs(QnnModelWrapper& qnn_model_wrapper,
                                                    const NodeUnit& node_unit,
                                                    std::vector<std::string>&& input_names,
@@ -79,22 +92,15 @@ Status SplitOpBuilder::ProcessAttributesAndOutputs(QnnModelWrapper& qnn_model_wr
       const int64_t* tensor_data = reinterpret_cast<const int64_t*>(unpacked_tensor.data());
       size_t tensor_byte_size = unpacked_tensor.size();
       size_t size = tensor_byte_size / sizeof(int64_t);
-      split_index.push_back(0);  // QNN need the start index of each range and starts from 0
-      std::transform(tensor_data, tensor_data + size, std::back_inserter(split_index),
-                     [](int64_t item) { return SafeInt<uint32_t>(item); });
-      split_index.pop_back();
+      ConvertSplitLengthsToSplitIndices({tensor_data, size}, split_index);
     } else {
       return ORT_MAKE_STATUS(ONNXRUNTIME, FAIL, "QNN doesn't support dynamic split");
     }
   } else {
     NodeAttrHelper node_helper(node_unit);
     if (node_helper.HasAttr("split")) {
-      auto split = node_helper.Get("split", std::vector<int32_t>{0});
-      uint32_t split_it = 0;
-      for (size_t i = 0; i < split.size(); ++i) {
-        split_index.push_back(split_it);
-        split_it += split[i];
-      }
+      auto split_lengths = node_helper.Get("split", std::vector<int64_t>{0});
+      ConvertSplitLengthsToSplitIndices(split_lengths, split_index);
     }
   }
 
@@ -105,11 +111,19 @@ Status SplitOpBuilder::ProcessAttributesAndOutputs(QnnModelWrapper& qnn_model_wr
                       "Cannot get shape");
     ORT_ENFORCE(static_cast<int32_t>(input_shape.size()) > axis_value, "axis not valid!");
     ORT_RETURN_IF_NOT(input_shape.at(axis_value) > 0, "Shape value not valid!");
-    auto num_outputs = node_unit.Outputs().size();
-    auto step = SafeInt<uint32_t>(input_shape.at(axis_value) / num_outputs);
+
+    // ONNX spec states that if not evenly divisible by `num_outputs`, the last chunk is smaller.
+    // Therefore, we have to use ceil() when computing shape[axis] / num_outputs.
+    // See: core/providers/cpu/tensor/split.cc::PrepareForCompute()
+    const float num_outputs = static_cast<float>(node_unit.Outputs().size());
+    const float split_dim_size = static_cast<float>(input_shape[axis_value]);
+    const uint32_t step = SafeInt<uint32_t>(std::ceil(split_dim_size / num_outputs));
     uint32_t split_it = 0;
+
     for (size_t i = 0; i < num_outputs; ++i) {
-      split_index.push_back(split_it);
+      if (i > 0) {  // 0th split index is implicit (QNN >= 2.19 raises validation error if included)
+        split_index.push_back(split_it);
+      }
       split_it += step;
     }
   }

onnxruntime/test/providers/qnn/split_op_test.cc

@@ -302,19 +302,46 @@ TEST_F(QnnHTPBackendTests, Split_Int32_Opset13) {
 // Test 8-bit QDQ Split opset 18 on HTP backend: equal split of axis 0 via 'num_outputs' attribute
 // and 'split' input.
 TEST_F(QnnHTPBackendTests, Split_Equal_Axis0_Opset18) {
+  // Split 6 into 3 outputs of lengths [2, 2, 2]
+  TestInputDef<float> input_def({6, 2}, false,
+                                {0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.f, 8.f, 9.0f, 10.0f, 11.0f});
+
   // Use 'split' input (initializer).
-  RunQDQSplitOpTestOnHTP<uint8_t>(TestInputDef<float>({4, 2}, false, {1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.f, 8.f}),
-                                  {2, 2},  // split
-                                  0,       // axis
-                                  -1,      // num_outputs
-                                  18,      // opset
+  RunQDQSplitOpTestOnHTP<uint8_t>(input_def,
+                                  {2, 2, 2},  // split
+                                  0,          // axis
+                                  -1,         // num_outputs
+                                  18,         // opset
                                   ExpectedEPNodeAssignment::All);
 
   // Use 'num_outputs' attribute.
-  RunQDQSplitOpTestOnHTP<uint8_t>(TestInputDef<float>({4, 2}, false, {1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.f, 8.f}),
+  RunQDQSplitOpTestOnHTP<uint8_t>(input_def,
+                                  {},  // split (use num_outputs instead)
+                                  0,   // axis
+                                  3,   // num_outputs
+                                  18,  // opset
+                                  ExpectedEPNodeAssignment::All);
+}
+
+// Test 8-bit QDQ Split opset 18 on HTP backend. Use an uneven split (last chunk should be smaller).
+TEST_F(QnnHTPBackendTests, Split_NonEqual_Axis0_Opset18) {
+  // Split 7 into 3 outputs of lengths [3, 3, 1]
+  TestInputDef<float> input_def({7, 2}, false,
+                                {0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.f, 8.f, 9.0f, 10.0f, 11.0f, 12.0f, 13.0f});
+
+  // Use a `split` input with uneven split lengths.
+  RunQDQSplitOpTestOnHTP<uint8_t>(input_def,
+                                  {3, 3, 1},  // split
+                                  0,          // axis
+                                  -1,         // num_outputs
+                                  18,         // opset
+                                  ExpectedEPNodeAssignment::All);
+
+  // Use a `num_outputs` attribute that does not evenly divide into shape[axis].
+  RunQDQSplitOpTestOnHTP<uint8_t>(input_def,
                                   {},  // split (use num_outputs instead)
                                   0,   // axis
-                                  2,   // num_outputs
+                                  3,   // num_outputs
                                   18,  // opset
                                   ExpectedEPNodeAssignment::All);
 }