[Development] Improve Codebase Quality

[ZihengJiang]

According to the development guidelines(https://github.com/octoml/relax/wiki/Relax-development-guidelines) proposed in the last open source meeting, we agree to improve the codebase quality with better error message and testing. I browsed the current codebase and feel there are lots of places can be improved under current guidelines, like

• In BlockBuilder, we should add example blocks for user interface like emit, function, etc.
• In many c++ code, we should have explicit error type for the error message;

I list the code owner for every part here which should be responsible for the code quality improvement before upstreaming under the new guideline (but others' help is definitely welcome)

• AST, VM, ExecBuilder, ShapeLowering @ZihengJiang
• VM CodeGen, Memory Allocator, BlockBuilder, ExprFunctor @YuchenJin
• Relax Script, Parser, ExprFunctor @altanh

Please correct me if I miss anything.

[tqchen]

Would be good to break this up into smaller issues, with actionable items.

[tqchen]

We have done a pass of refactoring of the components