[Fix] Detect intersection before using shapley.intersection to eliminate spurious warnings

[gaotongxiao]

Motivation

shapely raises a warning when users try to get the intersection of two non-overlapping polygons, resulting in overwhelming warnings in the first few training iterations when the text detection model cannot generate correct predictions and the metric tries to compute IoU between them and gts.

Close #1674

[codecov]

Codecov Report

Base: 88.32% // Head: 88.32% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (fcf8422) compared to base (2a2cab3).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head fcf8422 differs from pull request most recent head 7ade459. Consider uploading reports for the commit 7ade459 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1710   +/-   ##
========================================
  Coverage    88.32%   88.32%           
========================================
  Files          175      175           
  Lines        10896    10898    +2     
  Branches      1527     1528    +1     
========================================
+ Hits          9624     9626    +2     
  Misses         992      992           
  Partials       280      280           

Flag	Coverage Δ
unittests	88.32% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmocr/utils/polygon_utils.py	97.53% <100.00%> (+0.03%)	⬆️

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