Add outputLength method

Test: build succeeds, `atest libinput-tests` passes.

Bug: 268245099

Change-Id: I030da703a907eef44e85d186144eddc53b5998cc
diff --git a/include/input/TfLiteMotionPredictor.h b/include/input/TfLiteMotionPredictor.h
index 7de551b41..a340bd0 100644
--- a/include/input/TfLiteMotionPredictor.h
+++ b/include/input/TfLiteMotionPredictor.h
@@ -106,6 +106,9 @@
     // Returns the length of the model's input buffers.
     size_t inputLength() const;
 
+    // Returns the length of the model's output buffers.
+    size_t outputLength() const;
+
     // Executes the model.
     // Returns true if the model successfully executed and the output tensors can be read.
     bool invoke();