Add tryTo evalue accessors (#19039)

Add tryTo accessors for each value. Previously, `toTensor` etc. abort
with ET_CHECK_MSG when the type mismatches.

API additions:
- Per-type: tryToInt, tryToDouble, tryToBool, tryToScalar, tryToString,
  tryToTensor (already present, kept), tryToIntList, tryToBoolList,
  tryToDoubleList, tryToTensorList, tryToListOptionalTensor,
  tryToScalarType, tryToMemoryFormat, tryToLayout, tryToDevice.
  Tag mismatch returns Error::InvalidType; null list/string payload
  returns Error::InvalidState.
- Templated tryTo<T>() dispatcher mirroring to<T>(), via a new
  EVALUE_DEFINE_TRY_TO macro kept adjacent to EVALUE_DEFINE_TO so drift
  between the two surfaces is visible at review time.
- tryToOptional<T>() widened from Tensor-only to generic, delegating
  to tryTo<T>() so it works for any supported payload type.

Tests cover success + mismatch paths for each new accessor, plus the
widened tryToOptional<T>() path.

Authored-with: Claude

---------

Co-authored-by: Github Executorch <github_executorch@arm.com>

Author: lucylq

runtime/core/evalue.cpp

@@ -10,6 +10,10 @@
 
 namespace executorch {
 namespace runtime {
+
+// Specialize for list of optional tensors, as nullptr is a valid std::nullopt.
+// For non-optional types, nullptr is invalid.
+
 template <>
 executorch::aten::ArrayRef<std::optional<executorch::aten::Tensor>>
 BoxedEvalueList<std::optional<executorch::aten::Tensor>>::get() const {
@@ -27,5 +31,26 @@ BoxedEvalueList<std::optional<executorch::aten::Tensor>>::get() const {
   return executorch::aten::ArrayRef<std::optional<executorch::aten::Tensor>>{
       unwrapped_vals_, wrapped_vals_.size()};
 }
+
+template <>
+Result<executorch::aten::ArrayRef<std::optional<executorch::aten::Tensor>>>
+BoxedEvalueList<std::optional<executorch::aten::Tensor>>::tryGet() const {
+  for (typename executorch::aten::ArrayRef<
+           std::optional<executorch::aten::Tensor>>::size_type i = 0;
+       i < wrapped_vals_.size();
+       i++) {
+    if (wrapped_vals_[i] == nullptr) {
+      unwrapped_vals_[i] = std::nullopt;
+      continue;
+    }
+    auto r = wrapped_vals_[i]->tryToOptional<executorch::aten::Tensor>();
+    if (!r.ok()) {
+      return r.error();
+    }
+    unwrapped_vals_[i] = std::move(r.get());
+  }
+  return executorch::aten::ArrayRef<std::optional<executorch::aten::Tensor>>{
+      unwrapped_vals_, wrapped_vals_.size()};
+}
 } // namespace runtime
 } // namespace executorch