[MRG, BUG] Scan Opt fixes, enhencements

Add support for subsampling and gradient to GpuDnnConv

Add an assert, it fail if the post_condition aren't respected: that the remove constant and not used inputs is run before the save mem scan opt.

fix gh-1455 by making all version raise an error. (But not always the same error).

This also fix the problem where test_rop where randomly returning some
bad answer, but making that case return an error (0 step in scan). We still need to find why the scan save mem sometimes generated scan with 0 step.

_metadict is a dict that work with not hashable type. This is needed
for the SliceType.

This is needed for scan optimization to rebuild the node outside the scan.

FIX: tutorial pycuda <> theano compatibility

Warning

thunk needs lazy attribute