[mlir] Fix/clarify parts of MLIR toy tutorial chaper 4.
Summary:
* Let's use "override" when we're just doing standard baseclassing.
("Specialization" makes it sound like template specialization, which
this is not.)
* CallInterfaces.td has an include guard, so #ifdef not needed anymore.
* Omit duplicate code in code samples.
* Clarify which algorithm we're talking about.
* Mention that the ShapeInference code is code a snippet that belongs to
algorithm discussed in the paragraph above it.
* Add missing definition for createShapeInferencePass.
Differential Revision: https://reviews.llvm.org/D75260