Major environment refactoring (base version)

[cbhua]

Description

Please refer to the full environment refactor PR: https://github.com/ai4co/rl4co/pull/166.

Motivation and Context

Please refer to the full environment refactor PR: https://github.com/ai4co/rl4co/pull/166.

Types of changes

Compare with the full environment refactor, this base refactor only touches:

1. Environment structure;
2. Documentation updates;
3. Supporting generator;

And as a base refactor, this PR keeps:

1. All the data generate processes (size, default range, default distribution) are not modified;
2. All the step, reward calculation, etc logics are not modified;

Overall, this base version PR only works on moving code and doesn't change the logic for consequence. In the future version, we will refactor environments with the guide of the full refactor version step by step.

For more details, please refer to the full environment refactor PR: https://github.com/ai4co/rl4co/pull/166.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds core functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation (update in the documentation)
• [ ] Example (update in the folder of examples)

Checklist

• [x] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [x] I have updated the documentation accordingly.