Fix crash in AddSD.grad(). Move the c code to another op AddSD_ccode
Enable AddSD_ccode.

I changed how format is specified. There was no bug as the input
variable are put in the key. Now I make explicit in the op key the
format.

This simplify at a few places the code too.
MulSS and AddSS now support different input dtype.
Make AddSD_ccode work and tested.
Add smaller logo image for GSoC
Merge pull request #1727 from nouiz/sparse

Fix Sparse grad crash and implement mixed dtype in sparse Mul/Add