Export `local_conv1d`/`local_conv2d` in `tf.keras.backend`

This fix tries to address the issue raised in 25546 where
there is a mismatch between `tf.keras.backend` and `keras.backend`
for `local_conv1d`/`local_conv2d`.

This fix exports `local_conv1d`/`local_conv2d` in `tf.keras.backend`,
so that `tf.keras.backend` is compatible with `keras.backend`
(https://keras.io/backend/

).

This fix fixes 25546.

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