Add clone_instance for Module (#30168)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30168

Previous implementation of `clone` in `script::Module` copies both the module instance and the
class type, after we enabled type sharing https://github.com/pytorch/pytorch/pull/26666 we also
need to have a function to clone instance only and share the underlying class type.

Test Plan:
tbd

Imported from OSS

Differential Revision: D18631324

fbshipit-source-id: dbadcf19695faee0f755f45093b24618c047b9d1

test/cpp/jit/test_module_api.cpp

@@ -0,0 +1,45 @@
+#include <test/cpp/jit/test_base.h>
+#include <test/cpp/jit/test_utils.h>
+#include <torch/torch.h>
+
+namespace torch {
+namespace jit {
+
+using namespace torch::jit::script;
+
+void testModuleCloneInstance() {
+  auto cu = std::make_shared<CompilationUnit>();
+  auto cls = ClassType::create("foo.bar", cu, true);
+  auto attr_name = "attr";
+  cls->addAttribute(attr_name, IntType::get());
+  Module m(cu, cls);
+  auto v = IValue(2);
+  m.register_attribute(attr_name,
+                       IntType::get(),
+                       v,
+                       false);
+
+  Module m2 = m.clone();
+  Module m3 = m.clone_instance();
+  // Make sure copy works
+  ASSERT_EQ(m2.attr(attr_name).toInt(), 2);
+  ASSERT_EQ(m3.attr(attr_name).toInt(), 2);
+
+  // clone will copy both type and data, therefore we'll have a
+  // different type
+  ASSERT_NE(m.type(), m2.type());
+  // clone_instance only copies data, type is shared
+  ASSERT_EQ(m.type(), m3.type());
+
+  // change value of copied instance
+  m3.register_attribute(attr_name,
+                        IntType::get(),
+                        IValue(3),
+                        false);
+  // Verify value of original instance doesn't change
+  ASSERT_EQ(m2.attr(attr_name).toInt(), 2);
+  ASSERT_EQ(m3.attr(attr_name).toInt(), 3);
+}
+
+} // namespace jit
+} // namespace torch