Add GPU support and improve performance for tf.diag and tf.diag_part (#13666)

* improve tf.diag and tf.diag_part in CPU and GPU

* add comment

* make changes of DiagOp according to reviews

* tidy indent

* remove uesless comment prefix

* add shard function for DiagOp

* add benchmark for diag_op_test in core/kernel

* change symbol order in BUILD file

* remove empty line for Sanity Checks

* add some comments and fix benchmark throughput ratio for DiagOp