fix destroy_map bug in scan

add infer_shape method to Solve Op

initialize the destroy map to empty

No NEWS.txt. This is related to change in the trunk and happen only when we try to use Theano without g++.

Support for undefined gradients

Mixed3

(needed to express that gradient with respect to x is undefined
if x is allowed to be None)

added a new kind of bad grad, undefined grad

We don't want to do it at import time.

renamed FunctionGraph.extend to FunctionGraph.attach_feature

made extend call attach_feature

Tell how to get the full list.

No need to review in detail. This is sandbox stuff.