Introduce public MergedDataMap

Pull Request resolved: #14766

Add public merged data map. Module can use this to resolve multiple named data maps.
ghstack-source-id: 312935687
@exported-using-ghexport

Differential Revision: [D83527299](https://our.internmc.facebook.com/intern/diff/D83527299/)

Author: lucylq

extension/named_data_map/test/targets.bzl

@@ -8,6 +8,10 @@ def define_common_targets(is_fbcode=False):
             # intentionally don't work in xplat (since they're host-only tools).
             "ET_MODULE_ADD_MUL_DATA_PATH": "$(location fbcode//executorch/test/models:exported_program_and_data[ModuleAddMul.ptd])",
             "ET_MODULE_LINEAR_DATA_PATH": "$(location fbcode//executorch/test/models:exported_program_and_data[ModuleLinear.ptd])",
+<<<<<<< HEAD
+=======
+            "ET_MODULE_SIMPLE_TRAIN_DATA_PATH": "$(location fbcode//executorch/test/models:exported_program_and_data[ModuleSimpleTrain.ptd])",
+>>>>>>> 61affd021c (Introduce public MergedDataMap)
         }
 
         runtime.cxx_test(