Fixed a bug in pfunc ( bug =  when you replaced a shared variable with givens, the shared variable did not got replaced, and the update rule was executed ) that resulted in fixing the failing tests of scan. I also did a bit of cleaining in scan tests and code, and fix an unobserved bug in inplace computation of scan plus made sure scan knows (once the optimization is written) to only store the last k steps of an output

addition to Variables.

This was done so that the string could be passed to the logging module.

There's not a given bug this test reveals, but since it's written...

Makes test pass in FAST_COMPILE.

This makes the test pass with FAST_COMPILE

It makes no sense to have a softmax_grad node with inputs that are not
matrices of the same size. C implementation would have complained.
T_SoftmaxGrad.test_infer_shape now passes in FAST_COMPILE.