Do not load constant_segment if only the placeholder exists (pytorch#5229)

Summary:
Pull Request resolved: pytorch#5229

1. If constant segment offsets only contains one value, it is the placeholder value for non-const tensors. This means the constant segment is empty, and does not need to be loaded. (T201034228)

2. Clean up before returning error in `prepare_input_tensors`. (T201165663)

Differential Revision: D62455987

Author: lucylq

extension/runner_util/inputs.cpp

@@ -31,6 +31,7 @@ Result<BufferCleanup> prepare_input_tensors(Method& method) {
   for (size_t i = 0; i < num_inputs; i++) {
     auto tag = method_meta.input_tag(i);
     if (!tag.ok()) {
+      BufferCleanup cleanup({inputs, num_allocated});
       return tag.error();
     }
     if (tag.get() != Tag::Tensor) {

runtime/executor/program.cpp

@@ -150,9 +150,12 @@ Result<executorch_flatbuffer::ExecutionPlan*> get_execution_plan(
 
   // Constant data may live inside the flatbuffer data (constant_buffer) or in a
   // separate segment (constant_segment). It should not be in both.
+  // Check constant_segment->offsets()->size() > 1, as the offsets list will
+  // always contain a placeholder value 0 for non-const tensors. If this is the
+  // only offset, the constant segment is empty and does not need to be loaded.
   const auto* constant_segment = flatbuffer_program->constant_segment();
   if (constant_segment != nullptr && constant_segment->offsets() != nullptr &&
-      constant_segment->offsets()->size() > 0) {
+      constant_segment->offsets()->size() > 1) {
     // The constant data is inside a separate segment.
     const auto* constant_buffer = flatbuffer_program->constant_buffer();
     ET_CHECK_OR_RETURN_ERROR(