added a mathematical definition of Op.grad

 some fix for scan when computing gradients wrt to shared variables that where not passed to the inner function (added a new test in test_scan.py

a test where we generate an Elemwise composite with a mod that don't compile and commit the fix at the same time.

added a mechanisme to compute prin the size of memory allocated on the gpu if their is an error while allocating or free gpu memory.Disabled by default.