Update on "add a bunch of bounds checking to pytree"

It's possible to pass arbitrary string input to pytree from Python; let's not have a bunch of low-hanging memory safety issues.

Differential Revision: [D68166303](https://our.internmc.facebook.com/intern/diff/D68166303/)

[ghstack-poisoned]

Author: swolchok

extension/pytree/test/test_pytree.cpp

@@ -11,13 +11,28 @@
 #include <gtest/gtest.h>
 #include <string>
 
+using ::executorch::extension::pytree::arr;
 using ::executorch::extension::pytree::ContainerHandle;
 using ::executorch::extension::pytree::Key;
 using ::executorch::extension::pytree::Kind;
 using ::executorch::extension::pytree::unflatten;
 
 using Leaf = int32_t;
 
+TEST(PyTreeTest, ArrBasic) {
+  arr<int> x(5);
+  ASSERT_EQ(x.size(), 5);
+  EXPECT_THROW(x.at(5), std::out_of_range);
+  for (int ii = 0; ii < x.size(); ++ii) {
+    x[ii] = 2 * ii;
+  }
+  int idx = 0;
+  for (const auto item : x) {
+    EXPECT_EQ(item, 2 * idx);
+    ++idx;
+  }
+}
+
 TEST(PyTreeTest, List) {
   Leaf items[2] = {11, 12};
   std::string spec = "L2#1#1($,$)";