Throw in VK_GET_OP_FN if op is not found (pytorch#3028)

Summary:
Pull Request resolved: pytorch#3028

Make yipjustin happy. Forgot this safeguard when I originally wrote the `OperatorRegistry` class.

Reviewed By: SS-JIA

Differential Revision: D56085588

fbshipit-source-id: ba116eab8054e3610011fd0c8ffc0aabe61ae8ea

backends/vulkan/runtime/graph/ops/OperatorRegistry.cpp

@@ -16,7 +16,9 @@ bool OperatorRegistry::has_op(const std::string& name) {
 
 OperatorRegistry::OpFunction& OperatorRegistry::get_op_fn(
     const std::string& name) {
-  return table_.find(name)->second;
+  const auto it = table_.find(name);
+  VK_CHECK_COND(it != table_.end(), "Could not find operator with name ", name);
+  return it->second;
 }
 
 void OperatorRegistry::register_op(const std::string& name, OpFunction& fn) {