* [REFACTOR] relay::Module Def -> TypeDef

The term Def was not very clear about what is the object of interest(could be function def or type def).
Changes the term to TypeDef to be more explicit.

* Update include/tvm/relay/module.h

Co-Authored-By: Wei Chen <ipondering.weic@gmail.com>

Co-authored-by: Wei Chen <ipondering.weic@gmail.com>