Allow empty value for "_input_shapes" attr of functions in graphdef library

PiperOrigin-RevId: 360279759
Change-Id: I14edacb9aad6d4b995ef231520acad2b752b029d

tensorflow/compiler/mlir/tensorflow/tests/graphdef2mlir/empty-input-shapes.pbtxt

@@ -0,0 +1,39 @@
+# RUN: tf-mlir-translate -graphdef-to-mlir %s
+
+node {
+  name: "input"
+  op: "Placeholder"
+  attr {
+    key: "dtype"
+    value {
+      type: DT_BOOL
+    }
+  }
+}
+
+node {
+  name: "func0"
+  op: "func_name"
+  input: "input"
+}
+
+library {
+  function {
+    signature {
+      name: "func_name"
+      input_arg {
+        name: "arg0"
+        type: DT_BOOL
+      }
+    }
+    ret {
+      key: "retval0"
+      value: "arg0"
+    }
+    attr: {
+      key: "_input_shapes"
+      value: {
+      }
+    }
+  }
+}

tensorflow/compiler/mlir/tensorflow/translate/import_model.cc

@@ -1292,17 +1292,23 @@ Status ImporterBase::ConvertLibFunction(llvm::StringRef func_name) {
     if (name_and_value.first == "_input_shapes") {
       auto& list = name_and_value.second.list();
       auto& signature = func_def->signature();
-      if (list.shape_size() != signature.input_arg_size()) {
+      // Some models have "_input_shapes" attribute, but with its value empty
+      if (list.shape_size() > 0 &&
+          list.shape_size() != signature.input_arg_size()) {
         return errors::FailedPrecondition(
             "Number of input arguments must be equal to the length of "
             "_input_shapes attribute in function '",
             StringRefToView(func_name), "'.");
       }
-      for (int i = 0; i < list.shape_size(); i++) {
+      for (int i = 0; i < signature.input_arg_size(); i++) {
         auto& input_arg = signature.input_arg(i);
         auto& array_info = specs.inputs[input_arg.name()];
         array_info.imported_dtype = input_arg.type();
-        array_info.shape = list.shape(i);
+        // set to unranked for empty "_input_shapes" attribute
+        if (list.shape_size() > 0)
+          array_info.shape = list.shape(i);
+        else
+          array_info.shape.set_unknown_rank(true);
       }
     }
   }