- Replace existing implementation of aten.mean.dim with general version
invoking centralized `add_reduce_layer` utility
- Add implementation of aten.mean.default by refactoring default case to
be a special invocation of aten.mean.dim (specifically, one with `dim`
being all dimensions in the Tensor and `keepdim` being False).
- Add defaults for optional arguments in converters
- Add test cases for combinations of input options to the converters