added a fct to return the instance of the default mode. It check that it exist and raise an informative error if it don't exist.

In function_module there is a check in infer_reuse_pattern, which is wrong. I talked about it with James, he did a new version of infer_reuse_pattern and assumed that the new version should always return at least what the old version was returning. It turns out this is not the case and that the new version is correct.

removed Apply._creation_idx as this is not usefull anymore and was causing trouble with the unserialization of Apply node for the CompositeOp.

put the fusion of elemwise after the gemm optimizer as this make the gemm optimizer not able to make its work.