message: Add Grappler change release note
diff: * Add `keepdims` argument to all `GlobalPooling` layers.
  * `tf.lite`:
  * Fix mean op reference quantization rounding issue.
  +* `Grappler`:
  + * Disable default Grappler optimization timeout to make the optimization
  + pipeline deterministic. This may lead to increased model loading time,
  + because time spent in graph optimizations is now unbounded (was 20
  + minutes).
  ## Thanks to our Contributors
  