Fix wrong __len__() of Subset when the item is removed

[vinnamkim]

Summary

• @sungmanc reported that the subset length is not correctly obtained if we remove the item from the dataset as follows.
  
• Ticket no. 105934

How to test

Added a test for this bug

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.08 ⚠️

Comparison is base (21755d3) 78.59% compared to head (3b1dc83) 78.51%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #854      +/-   ##
===========================================
- Coverage    78.59%   78.51%   -0.08%     
===========================================
  Files          198      194       -4     
  Lines        24377    24268     -109     
  Branches      4932     4914      -18     
===========================================
- Hits         19159    19055     -104     
+ Misses        4118     4116       -2     
+ Partials      1100     1097       -3     

Flag	Coverage Δ
macos-11_Python-3.8	?
ubuntu-20.04_Python-3.8	?
windows-2019_Python-3.8	78.51% <66.66%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
datumaro/components/dataset.py	82.14% <66.66%> (-0.28%)	⬇️

... and 6 files with indirect coverage changes

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[sungmanc]

Thanks for quick fix.

[sooahleex]

Looks good to me.