[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