Pass dtype to constructor in LSTMCell (#18178)

* Use float32 in case the dtype is not set in the constructor

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

* Add test case for 16228.

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

* Add test case where dype is passed explicitly.

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

* Fix pylint issue

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

* Replace strings to objects to address review feedback.

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