[Doc] Fix docstring warnings

[jin-s13]

Motivation

Fix most of the docstring warnings. Make mmpose.api look good in the doc.
Check https://mmpose--1084.org.readthedocs.build/en/1084/api.html

Modification

1. Take care of the spelling. Use "Note:" instead of "Notes:". Use "Returns:" instead of "return:"
2. Use "-" for each item in the list.

For example:

    Note:
        - num_bboxes: N
        - num_keypoints: K

3. Use a "tab" (4 spaces) before a new line.
4. Add an empty line before bullet list.
5. Use lower case for "list" and "tuple", instead of "List" and "Tuple".
6. Multiple lines in "Returns", should add "" at the end of each line.
7. For "Returns", if multiple variables are returned, the format should look like this..

  Returns:
        tuple: A tuple containing multi-stage outputs.

        - list[Tensor]: multi-stage heatmaps.
        - list[Tensor]: multi-stage tags.

8. Other Format

The example below does not look good.

Returns:
        flipped_feature_maps (list(torch.Tensor)): Flipped feature_maps.

Please consider modifying it as follows. (1) Remove the variable name (2) change () -> []

Returns:
        list[Tensor]: Flipped feature_maps.

9. Please use "," to separate Tensor/np.ndarray shape instead of using "x"

torch.Tensor[NxCxHxW] --> torch.Tensor[N,C,H,W]

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.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev-0.22@c918dbe). Click here to learn what that means.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             dev-0.22    #1084   +/-   ##
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  Coverage            ?   82.42%           
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  Files               ?      202           
  Lines               ?    16032           
  Branches            ?     2860           
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  Hits                ?    13215           
  Misses              ?     2095           
  Partials            ?      722           

Flag	Coverage Δ
unittests	82.36% <0.00%> (?)

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