[JIT] Fix getting type for subscript assignments. (pytorch#106041)

### Description

Hi! We've been fuzzing `pytorch` with [sydr-fuzz](https://github.com/ispras/oss-sydr-fuzz) and found error of out of bounds access in `torch::jit` module.

pytorch version: 18bcf62

The error occurs in `import_source.cpp:560` when we get the type from the `assign.rhs()`. `assign.rhs()` has `Maybe` type, as well as `assign.type()`, so one of them can be not presented. According to [grammar](https://github.com/pytorch/pytorch/blob/22f93852a2664b3dc29544ac6a36f1ec52c6caa2/torch/csrc/jit/frontend/tree_views.h), we can have `Assign` statement, which `lhs` will be `Subscript`, `rhs` will be empty (`Maybe` type with no subtrees) and `type` will be presented. But in `import_source.cpp:560` we try to get `rhs` expression from the assignment with no check whether it is presented.

This is example from the how to reproduce section from the testing input:
```
class Module(Module):
  __parameters__ = ["0", ]
  __buffers__ = []
  __annotations__ = []
  __annotations__["0"] : Tensor
```

When we parse the last statement of class definition, we set the type of `lhs` to `Subscript`, because the lookahead is `[`
https://github.com/pytorch/pytorch/blob/76fb72e24a5a4a47ad1f50c5c94d5c0b7e703531/torch/csrc/jit/frontend/parser.cpp#L205-L207

Then in `parseAssignment` we get `maybeOp` and `type` depending on the next symbol (if it is `:`, we get only the type)
https://github.com/pytorch/pytorch/blob/76fb72e24a5a4a47ad1f50c5c94d5c0b7e703531/torch/csrc/jit/frontend/parser.cpp#L437-L447

So after that, in `import_source.cpp:560`, parsing attributes, one of which is assignment with subscript type of `lhs`, we try to get type from `rhs` expression and out of bounds access occurs.

To fix the error, we need to check whether the `rhs` or `type` are presented and get the type from corresponding expression.

### How to reproduce

Build docker container from [here](https://github.com/ispras/oss-sydr-fuzz/tree/master/projects/pytorch):
```bash
$ sudo docker build -t oss-sydr-fuzz-pytorch
```

Run docker container:
```bash
$ sudo docker run --rm --privileged -v `pwd`:/fuzz -it oss-sydr-fuzz-pytorch /bin/bash
```

Run the `load_fuzz` target on the [input.txt](https://github.com/pytorch/pytorch/files/12173962/input.txt)
```bash
/load_fuzz input.txt
```

You will see the following output:
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==157==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x00000c163764 bp 0x7ffee71d0070 sp 0x7ffee71d0050 T0)
==157==The signal is caused by a READ memory access.
==157==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0xc163764 in c10::intrusive_ptr<torch::jit::Tree, c10::detail::intrusive_target_default_null_type<torch::jit::Tree> >::retain_() /pytorch/c10/util/intrusive_ptr.h:265:54
    #1 0xc1697fd in c10::intrusive_ptr<torch::jit::Tree, c10::detail::intrusive_target_default_null_type<torch::jit::Tree> >::intrusive_ptr(c10::intrusive_ptr<torch::jit::Tree, c10::detail::intrusive_target_default_null_type<torch::jit::Tree> > const&) /pytorch/c10/util/intrusive_ptr.h:354:5
    #2 0xc1697fd in torch::jit::Expr::Expr(c10::intrusive_ptr<torch::jit::Tree, c10::detail::intrusive_target_default_null_type<torch::jit::Tree> > const&) /pytorch/torch/csrc/jit/frontend/tree_views.h:270:49
    #3 0xc1f02cb in torch::jit::Maybe<torch::jit::Expr>::get() const /pytorch/torch/csrc/jit/frontend/tree_views.h:212:12
    #4 0xd194369 in torch::jit::SourceImporterImpl::importClass(c10::QualifiedName const&, torch::jit::ClassDef const&, bool) /pytorch/torch/csrc/jit/serialization/import_source.cpp:560:70
    #5 0xd18c701 in torch::jit::SourceImporterImpl::importNamedType(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, torch::jit::ClassDef const&) /pytorch/torch/csrc/jit/serialization/import_source.cpp:288:5
    pytorch#6 0xd18a84c in torch::jit::SourceImporterImpl::findNamedType(c10::QualifiedName const&) /pytorch/torch/csrc/jit/serialization/import_source.cpp:140:5
    pytorch#7 0xd1913a8 in torch::jit::SourceImporterImpl::resolveType(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, torch::jit::SourceRange const&) /pytorch/torch/csrc/jit/serialization/import_source.cpp:261:10
    pytorch#8 0xc2e422f in torch::jit::ScriptTypeParser::parseTypeFromExpr(torch::jit::Expr const&) const /pytorch/torch/csrc/jit/frontend/script_type_parser.cpp:238:24
    pytorch#9 0xc2e4697 in torch::jit::ScriptTypeParser::parseType(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /pytorch/torch/csrc/jit/frontend/script_type_parser.cpp:312:10
    pytorch#10 0xd1a37d4 in torch::jit::SourceImporter::loadType(c10::QualifiedName const&) const /pytorch/torch/csrc/jit/serialization/import_source.cpp:786:27
    pytorch#11 0xd121c47 in torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0::operator()(c10::QualifiedName const&) const /pytorch/torch/csrc/jit/serialization/import.cpp:146:33
    pytorch#12 0xd121c47 in c10::StrongTypePtr std::__invoke_impl<c10::StrongTypePtr, torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0&, c10::QualifiedName const&>(std::__invoke_other, torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0&, c10::QualifiedName const&) /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/invoke.h:60:14
    pytorch#13 0xd121ad0 in std::enable_if<is_invocable_r_v<c10::StrongTypePtr, torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0&, c10::QualifiedName const&>, c10::StrongTypePtr>::type std::__invoke_r<c10::StrongTypePtr, torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0&, c10::QualifiedName const&>(torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0&, c10::QualifiedName const&) /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/invoke.h:113:9
    pytorch#14 0xd121926 in std::_Function_handler<c10::StrongTypePtr (c10::QualifiedName const&), torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)::$_0>::_M_invoke(std::_Any_data const&, c10::QualifiedName const&) /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/std_function.h:291:9
    pytorch#15 0xd17ec49 in std::function<c10::StrongTypePtr (c10::QualifiedName const&)>::operator()(c10::QualifiedName const&) const /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/std_function.h:622:14
    pytorch#16 0xd26b802 in torch::jit::Unpickler::readGlobal(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /pytorch/torch/csrc/jit/serialization/unpickler.cpp:844:9
    pytorch#17 0xd2615fb in torch::jit::Unpickler::readInstruction() /pytorch/torch/csrc/jit/serialization/unpickler.cpp:520:7
    pytorch#18 0xd25f917 in torch::jit::Unpickler::run() /pytorch/torch/csrc/jit/serialization/unpickler.cpp:253:27
    pytorch#19 0xd25f5b2 in torch::jit::Unpickler::parse_ivalue() /pytorch/torch/csrc/jit/serialization/unpickler.cpp:206:3
    pytorch#20 0xd186403 in torch::jit::readArchiveAndTensors(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, c10::optional<std::function<c10::StrongTypePtr (c10::QualifiedName const&)> >, c10::optional<std::function<c10::intrusive_ptr<c10::ivalue::Object, c10::detail::intrusive_target_default_null_type<c10::ivalue::Object> > (c10::StrongTypePtr, c10::IValue)> >, c10::optional<c10::Device>, caffe2::serialize::PyTorchStreamReader&, c10::Type::SingletonOrSharedTypePtr<c10::Type> (*)(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&), std::shared_ptr<torch::jit::DeserializationStorageContext>) /pytorch/torch/csrc/jit/serialization/import_read.cpp:53:20
    pytorch#21 0xd12152d in torch::jit::(anonymous namespace)::ScriptModuleDeserializer::readArchive(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /pytorch/torch/csrc/jit/serialization/import.cpp:184:10
    pytorch#22 0xd117bae in torch::jit::(anonymous namespace)::ScriptModuleDeserializer::deserialize(c10::optional<c10::Device>, std::unordered_map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >&, bool) /pytorch/torch/csrc/jit/serialization/import.cpp:287:19
    pytorch#23 0xd114074 in torch::jit::import_ir_module(std::shared_ptr<torch::jit::CompilationUnit>, std::istream&, c10::optional<c10::Device>, std::unordered_map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::hash<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >&, bool, bool) /pytorch/torch/csrc/jit/serialization/import.cpp:389:25
    pytorch#24 0xd113a27 in torch::jit::import_ir_module(std::shared_ptr<torch::jit::CompilationUnit>, std::istream&, c10::optional<c10::Device>, bool) /pytorch/torch/csrc/jit/serialization/import.cpp:325:10
    pytorch#25 0xd11bb64 in torch::jit::load(std::istream&, c10::optional<c10::Device>, bool) /pytorch/torch/csrc/jit/serialization/import.cpp:485:10
    pytorch#26 0x610c5c in LLVMFuzzerTestOneInput /load.cc:42:14
    pytorch#27 0x537701 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /llvm-project-llvmorg-14.0.6/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    pytorch#28 0x52160c in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /llvm-project-llvmorg-14.0.6/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:324:6
    pytorch#29 0x52735b in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /llvm-project-llvmorg-14.0.6/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:860:9
    pytorch#30 0x550912 in main /llvm-project-llvmorg-14.0.6/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    pytorch#31 0x7f06e8323082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)
    pytorch#32 0x51bf2d in _start (/load_fuzz+0x51bf2d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /pytorch/c10/util/intrusive_ptr.h:265:54 in c10::intrusive_ptr<torch::jit::Tree, c10::detail::intrusive_target_default_null_type<torch::jit::Tree> >::retain_()
==157==ABORTING
```
Pull Request resolved: pytorch#106041
Approved by: https://github.com/davidberard98

torch/csrc/jit/serialization/import_source.cpp

@@ -549,15 +549,19 @@ void SourceImporterImpl::importClass(
       case TK_VAR: {
         const auto name = Var(assign.lhs()).name().name();
         TORCH_INTERNAL_ASSERT(name != "__parameters__");
-        const auto type = type_parser.parseTypeFromExpr(assign.type().get());
+        const auto type = assign.type().present()
+            ? type_parser.parseTypeFromExpr(assign.type().get())
+            : type_parser.parseTypeFromExpr(assign.rhs().get());
         const bool is_parameter = parameter_names.count(name);
         const bool is_buffer = buffer_names.count(name);
         class_type->addAttribute(name, type, is_parameter, is_buffer);
       } break;
       case TK_SUBSCRIPT: {
         const auto name =
             StringLiteral(Subscript(assign.lhs()).subscript_exprs()[0]).text();
-        const auto type = type_parser.parseTypeFromExpr(assign.rhs().get());
+        const auto type = assign.type().present()
+            ? type_parser.parseTypeFromExpr(assign.type().get())
+            : type_parser.parseTypeFromExpr(assign.rhs().get());
         const bool is_parameter = parameter_names.count(name);
         const bool is_buffer = buffer_names.count(name);
         class_type->addAttribute(name, type, is_parameter, is_buffer);