Fixed grad of grad of scan

Fixed tests with script run_tests_in_batch.py

Note sure it makes the file anymore readable, but at least I've tried.

Otherwise the numberic differentiation gradient quickly becomes numerically
unstable.

the index goes from 0 to n_mit_mot-1 for mit_mot sequnces.

The main bug was gradients where represented as shared variables.
Now we represent them as sit_sot sequences to which only the last step
is used (hence the savemem optimization does the memory clean up). The
advantage is that gradients with respect to sitsot are well defined.