fix batch_dot tests on backend
fix batch_dot tests on backend

* Fix merge_dot tests

* Make batch_dot unique

batch_dot is not tensordot! It only accepts one reduce dimension at a
time. Other reduce dimensions should be dome afterwards with K.sum
This means that K.batch_dot will have the same behavior in both
tensorflow and theano. This also means that we have less parenthesis and
less nested lists.

New usage:

merge_mode = 'dot', dot_axes=[axis1, axis2]

Before:

merge_mode = 'dot', dot_axes=[[axis1], [axis2]]

* Backport sign by @the-moliver

* Fix docstrings

* Fix backend batch_dot tests

kemaswill/keras,nebw/keras,dolaameng/keras,keras-team/keras,keras-team/keras,relh/keras,DeepGnosis/keras,kuza55/keras