Adjust MethodMeta's non-const indices by 1, and update all users (#315)

Summary:
Pull Request resolved: #315

First step in hiding the off-by-one non-const buffer indices from users. Since fewer users use `MethodMeta::num_non_const_buffers` today (relative to the version on Program), update the behavior here, along with all of its users.

After this, we can migrate the users of `Program::get_non_const_buffer_size` to this new API, fixing the off-by-one as we go.

I found all of these uses with the search

```
cd fbsource
xbgs -sl "non_const_buffer_size" | xargs grep -le '\<non_const_buffer_size\>'
```

Luckily, `Program`'s version of this is called `get_non_const_buffer_size` (with the extra `get_` prefix) so it was straightforward to find code that used `MethodMeta`'s methods.

Reviewed By: JacobSzwejbka

Differential Revision: D48762563

fbshipit-source-id: bd5b1b7dc7702cf0192261b535d59e8ba80b96e4

Author: dbort

exir/backend/test/demos/rpc/ExecutorBackend.cpp

@@ -77,20 +77,19 @@ class ExecutorBackend final : public PyTorchBackendInterface {
         runtime_allocator, MemoryAllocator);
     new (client_const_allocator) MemoryAllocator(0, nullptr);
 
-    auto num_buffers = method_meta->num_non_const_buffers();
-    size_t num_non_const_buffers = num_buffers - 1;
+    auto num_non_const_buffers = method_meta->num_non_const_buffers();
 
     uint8_t** non_const_buffers = ET_ALLOCATE_LIST_OR_RETURN_ERROR(
         runtime_allocator, uint8_t*, num_non_const_buffers);
     MemoryAllocator* non_const_allocators = ET_ALLOCATE_LIST_OR_RETURN_ERROR(
         runtime_allocator, MemoryAllocator, num_non_const_buffers);
 
-    for (size_t id = 1; id < num_buffers; ++id) {
+    for (size_t id = 0; id < num_non_const_buffers; ++id) {
       auto buffer_size = method_meta->non_const_buffer_size(id);
       uint8_t* buffer_i = ET_ALLOCATE_LIST_OR_RETURN_ERROR(
           runtime_allocator, uint8_t, buffer_size.get());
-      non_const_buffers[id - 1] = buffer_i;
-      new (&non_const_allocators[id - 1])
+      non_const_buffers[id] = buffer_i;
+      new (&non_const_allocators[id])
           MemoryAllocator(static_cast<uint32_t>(buffer_size.get()), buffer_i);
     }
 

extension/pybindings/pybindings.cpp

@@ -83,8 +83,7 @@ class Module final {
     for (size_t i = 0; i < program_->num_methods(); ++i) {
       auto name = program_->get_method_name(i).get();
       auto method_meta = program_->method_meta(name).get();
-      // 1 on purpose because non-const are 1 indexed
-      for (size_t j = 1; j < method_meta.num_non_const_buffers(); j++) {
+      for (size_t j = 0; j < method_meta.num_non_const_buffers(); j++) {
         int64_t buffer_size = method_meta.non_const_buffer_size(j).get();
         if (non_const_buffer_sizes.find(j) == non_const_buffer_sizes.end()) {
           non_const_buffer_sizes.insert({j, buffer_size});

runtime/executor/method_meta.cpp

@@ -170,7 +170,10 @@ Result<TensorInfo> MethodMeta::output_tensor_meta(size_t index) const {
 }
 
 size_t MethodMeta::num_non_const_buffers() const {
-  return s_plan_->non_const_buffer_sizes()->size();
+  // Index zero is reserved internally, and we hide it from users. The actual
+  // number of buffers is one fewer than the actual size of this list in the
+  // program.
+  return s_plan_->non_const_buffer_sizes()->size() - 1;
 }
 
 Result<int64_t> MethodMeta::non_const_buffer_size(size_t index) const {
@@ -181,7 +184,9 @@ Result<int64_t> MethodMeta::non_const_buffer_size(size_t index) const {
       "index %zu out of range. num_buffers: %zu",
       index,
       num_buffers);
-  return s_plan_->non_const_buffer_sizes()->Get(index);
+  // Index zero is reserved internally, and we hide it from users. Adjust the
+  // provided index to point to one of the actual buffers.
+  return s_plan_->non_const_buffer_sizes()->Get(index + 1);
 }
 
 } // namespace executor

runtime/executor/test/method_meta_test.cpp

@@ -76,14 +76,14 @@ TEST_F(MethodMetaTest, MethodMetaApi) {
   EXPECT_EQ(method_meta->num_outputs(), 1);
 
   // Appropriate amount of non_const_buffers
-  EXPECT_EQ(method_meta->num_non_const_buffers(), 2);
+  EXPECT_EQ(method_meta->num_non_const_buffers(), 1);
 
   // Appropriate content of non_const_buffers
-  EXPECT_EQ(method_meta->non_const_buffer_size(1).get(), 48);
+  EXPECT_EQ(method_meta->non_const_buffer_size(0).get(), 48);
 
   // Invalid index Errors
   EXPECT_EQ(
-      method_meta->non_const_buffer_size(2).error(), Error::InvalidArgument);
+      method_meta->non_const_buffer_size(1).error(), Error::InvalidArgument);
 
   EXPECT_EQ(
       program_->method_meta("not_a_method").error(), Error::InvalidArgument);