Fix GRUBlockCell parameter naming inconsistency (#13153)

* Fix GRUBlockCell parameter naming inconsistency

This fix tries to fix the issue in 13137 where
parameter `cell_size` is used instead of `num_units`.
This is inconsistent with other RNN cells.

This fix adds support of `num_units` while at the same
time maintains backward compatiblility for `cell_size`.

This fix fixes 13137.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add `@deprecated_args` for 'cell_size' in `GRUBlockCell`

This commit adds `@deprecated_args` for 'cell_size' in `GRUBlockCell`

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Address review comment

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>