Spark/Lightning: fix checkpoint callback dirpath typo (#3204)

Signed-off-by: Chongxiao Cao <chongxiaoc@uber.com>

horovod/spark/lightning/remote.py

@@ -132,7 +132,7 @@ def train(serialized_model):
             # Otherwise we are seeing hanging in training.
             for cb in callbacks:
                 if isinstance(cb, ModelCheckpoint):
-                    cb.dir_path = ckpt_dir
+                    cb.dirpath = ckpt_dir
                     cb.filename = ckpt_filename
                     _checkpoint_callback = cb
                     require_checkpoint = True