Corrected DEBUG_MODE and FAST_COMPILE tests for grad of Prod. Added comments on mechanism for this grad.

fix may_share_memory for sparse type as it was done for ndarray type. Add may_share_memory test for CudaNdarray type

Allow the default value of parameter dx,dy of ConvOp.__init__. They are set explicitly to 1 by the conv() fct.

remove FAST_RUN_NOGC from the acceptable value of flag mode. People should use the flag linker=c|py_nogc

In a test, test that we generate an error only when one is needed and test the type of the error generated.