Create directory automatically in save_weights with h5 format

This fix tries to address the issue raised in 25835 where
save_weight with tf format creates directories automatically
as needed, yet h5 format throws out excetion.

This fix update the save_weight with h5 format to address
the discrepancy between h5 and tf.

This fix fixes 25835.

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