fix: Refactor implementation to remove nullptr

- Edit in favor of `c10::optional` type usage

Author: gs-olive

core/lowering/passes/remove_unnecessary_casts.cpp

@@ -218,10 +218,10 @@ const std::unordered_set<c10::Symbol> AtenIntReplacementNodeKinds = {
     torch::jit::aten::floor_divide,
 };
 
-torch::jit::Value* Validate0DTensor(torch::jit::Value* value) {
+c10::optional<torch::jit::Value*> Validate0DTensor(torch::jit::Value* value) {
   // Validates that the input Value* is a 0D Tensor (or int/float)
   // Return the stored int/float Value* if so, otherwise null
-  torch::jit::Value* enclosed_scalar_value = nullptr;
+  c10::optional<torch::jit::Value*> enclosed_scalar_value = {};
 
   // Regular Int/Float case
   if (value->type()->isSubtypeOf(c10::IntType::get()) || value->type()->isSubtypeOf(c10::FloatType::get())) {
@@ -257,7 +257,7 @@ torch::jit::Value* Validate0DTensor(torch::jit::Value* value) {
   return enclosed_scalar_value;
 }
 
-torch::jit::Value* TracebackAndEliminate0DTensors(torch::jit::Node* node) {
+c10::optional<torch::jit::Value*> TracebackAndEliminate0DTensors(torch::jit::Node* node) {
   // Trace back through a node and all parents to eliminate 0D Tensors
   // and update schemas to their scalar alternatives, returning final
   // Value* to user
@@ -268,30 +268,30 @@ torch::jit::Value* TracebackAndEliminate0DTensors(torch::jit::Node* node) {
     LOG_DEBUG(
         "Encountered node " << node->kind().toQualString()
                             << " which is unsupported in the aten::Int.Tensor replacement lowering pass.");
-    return nullptr;
+    return {};
   }
 
   // Validate the first and second function inputs are 0D tensors or scalars
-  torch::jit::Value* first_input_scalar_value = Validate0DTensor(node->inputs()[0]);
-  torch::jit::Value* second_input_scalar_value = Validate0DTensor(node->inputs()[1]);
+  c10::optional<torch::jit::Value*> first_input_scalar_value = Validate0DTensor(node->inputs()[0]);
+  c10::optional<torch::jit::Value*> second_input_scalar_value = Validate0DTensor(node->inputs()[1]);
 
   // If the first input is not a scalar, recursively traceback on parent nodes
-  if (!first_input_scalar_value) {
+  if (!first_input_scalar_value.has_value()) {
     LOG_DEBUG("In aten::Int.Tensor lowering, now tracing " << node->inputs()[0]->node()->kind().toQualString());
     first_input_scalar_value = TracebackAndEliminate0DTensors(node->inputs()[0]->node());
   }
 
   // If the second input is not a scalar, recursively traceback on parent nodes
-  if (!second_input_scalar_value) {
+  if (!second_input_scalar_value.has_value()) {
     LOG_DEBUG("In aten::Int.Tensor lowering, now tracing " << node->inputs()[0]->node()->kind().toQualString());
     second_input_scalar_value = TracebackAndEliminate0DTensors(node->inputs()[1]->node());
   }
 
-  if (!first_input_scalar_value || !second_input_scalar_value) {
+  if (!first_input_scalar_value.has_value() || !second_input_scalar_value.has_value()) {
     LOG_DEBUG(
         "In aten::Int.Tensor lowering, recursive trace through node input "
         << "parents failed to return a Scalar value for at least one parent node.");
-    return nullptr;
+    return {};
   }
 
   // Set default insert point at node
@@ -303,15 +303,16 @@ torch::jit::Value* TracebackAndEliminate0DTensors(torch::jit::Node* node) {
     // must be inserted
     case torch::jit::aten::floor_divide:
       new_node = node->owningGraph()->create(
-          torch::jit::aten::floordiv, {first_input_scalar_value, second_input_scalar_value}, 1);
+          torch::jit::aten::floordiv, {first_input_scalar_value.value(), second_input_scalar_value.value()}, 1);
       new_node->insertAfter(node);
       new_node->output()->setType(c10::IntType::get());
       return new_node->output();
 
     // In the aten::mul case, the schema syntax is the same, so we can use the existing schema
     // with new inputs
     default:
-      new_node = node->owningGraph()->create(node->kind(), {first_input_scalar_value, second_input_scalar_value}, 1);
+      new_node = node->owningGraph()->create(
+          node->kind(), {first_input_scalar_value.value(), second_input_scalar_value.value()}, 1);
       new_node->insertAfter(node);
       new_node->output()->setType(c10::IntType::get());
       return new_node->output();
@@ -336,8 +337,8 @@ void ReplaceAtenInt(std::shared_ptr<torch::jit::Graph>& g) {
             "Tracing parent node " << it->input()->node()->kind().toQualString()
                                    << " to eliminate 0D Tensors for aten::Int.Tensor case.");
         auto scalar_input_value = TracebackAndEliminate0DTensors(it->input()->node());
-        if (scalar_input_value) {
-          it->output()->replaceAllUsesWith(scalar_input_value);
+        if (scalar_input_value.has_value()) {
+          it->output()->replaceAllUsesWith(scalar_input_value.value());
           LOG_DEBUG("Tracing parent nodes for aten::Int.Tensor case succeeded.");
         } else {
           LOG_DEBUG("Tracing parent nodes for aten::Int.Tensor case failed.");