[Refactor] Modify some scripts to fit MultilevelPixelData

[liqikai9]

Motivation

1. Unify the multiscale, multi-scale, multi-level to multilevel; add multilevel_gt_fields in PoseDataSample;
2. Modify MSPNHead to fit MultilevelPixelData;
3. Fix some bugs in MultilevelPixelData add unittests for MultilevelPixelData;
4. Modify the MSPNd and RSN configs according to the above modificatios.

All unitetsts passed locally.
The test accuracy for configs/body_2d_keypoint/topdown_heatmap/coco/td-hm_2xmspn50_8xb32-210e_coco-256x192.py is verified, will also verify other modified configs.

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[ly015]

Please rebase onto the latest dev-1.x branch and fix conflicts. Also please check it can pass all unit tests.

[liqikai9]

Please rebase onto the latest dev-1.x branch and fix conflicts. Also please check it can pass all unit tests.

Done.