[Refactor] Complete the tests and docs for Python frontend

[seanlatias]

In this PR, the following changes are made.

APIs

• hcl.var is now deprecated and should not be used anymore
• Separate compute APIs (e.g., compute, update) from build APIs (e.g., lower, schedule, build)
• Move modules to be a part of the imperative DSL
• Allow users to use hcl.return_ with hcl.compute
• Add proper checks for the correctness of the compute APIs
• Users can define more complicated reduction functions with hcl.return_
• Now generate proper IR for reductions with multi-dimensional accumulators
• Rename hcl.module to hcl.def_
• Allow users to specify the data type of a for loop variable
• Add proper checks for if_, else_, and elif_

Docs

• Specify the orders for APIs in the docs
• Add autosummary for certain APIs
• Remove the intermediate page between the index and the tutorials
• Add Pylint specification
• All frontend APIs pass the Lint

Tests

• Add tests for hcl.compute, hcl.update, and hcl.mutate
• Add tests for reductions
• Add tests for imperative DSLs

Developer

• Complete the support for mutators (remove the decorator for better inheritance)

TODO

• Add tests for incorrect usages