[Bug] Fix heatmap and tag size mismatch in bottom-up with udp

[wusize]

Motivation

Tag-map size and heatmap size are not aligned for udp in bottom-up approaches.

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.

[codecov]

Codecov Report

Merging #994 (2b9867d) into master (2b4a5cf) will decrease coverage by 0.01%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master     #994      +/-   ##
==========================================
- Coverage   82.72%   82.70%   -0.02%     
==========================================
  Files         190      190              
  Lines       14190    14197       +7     
  Branches     2439     2441       +2     
==========================================
+ Hits        11739    11742       +3     
- Misses       1828     1832       +4     
  Partials      623      623              

Flag	Coverage Δ
unittests	82.63% <28.57%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmpose/models/detectors/associative_embedding.py	72.72% <28.57%> (-2.48%)	⬇️
mmpose/datasets/pipelines/shared_transform.py	87.56% <0.00%> (+0.49%)	⬆️

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