Do not return SignatureDef.defaults from GetModelMetadata

`GetModelMetadata` is expected to be fast and returns only the (small) metadata of the model. Returning the default values violates this expectation as default value tensors may be arbitrarily big.

PiperOrigin-RevId: 764009019

Author: junwhanahn

tensorflow_serving/servables/tensorflow/tfrt_servable.cc

@@ -260,8 +260,12 @@ absl::Status TfrtSavedModelServable::GetModelMetadata(
       SignatureDefMap signature_def_map;
       for (const auto& signature :
            saved_model_->GetMetaGraphDef().signature_def()) {
-        (*signature_def_map.mutable_signature_def())[signature.first] =
-            signature.second;
+        // Explicitly avoid copying `SignatureDef.defaults` as they can be big.
+        SignatureDef& def =
+            (*signature_def_map.mutable_signature_def())[signature.first];
+        def.set_method_name(signature.second.method_name());
+        *def.mutable_inputs() = signature.second.inputs();
+        *def.mutable_outputs() = signature.second.outputs();
       }
 
       auto* response_model_spec = response->mutable_model_spec();