Removing extra token classification metrics in backend

[frascuchon]

Description

The purpose of this PR is to reduce the number of generated properties used for token classification metrics computation. Sometimes these extra properties raise index errors as described in issue #1499

With these changes, the tokens_length, token_lenght, mention_lengh, and entity_density metrics will return no data from new servers.

Refs #1499

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

Current tests have been adapted to the changes

Checklist

• I have merged the original branch into my forked branch
• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.11 ⚠️

Comparison is base (22511cc) 90.86% compared to head (3933fbf) 90.75%.

❗ Current head 3933fbf differs from pull request most recent head 0894916. Consider uploading reports for the commit 0894916 to get more accurate results

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/1.8.0    #3045      +/-   ##
==================================================
- Coverage           90.86%   90.75%   -0.11%     
==================================================
  Files                 208      208              
  Lines               11096    11067      -29     
==================================================
- Hits                10082    10044      -38     
- Misses               1014     1023       +9     

Flag	Coverage Δ
pytest	90.75% <100.00%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
...erver/daos/backend/metrics/token_classification.py	100.00% <ø> (ø)
...erver/services/tasks/token_classification/model.py	98.00% <ø> (-1.02%)	⬇️
...rc/argilla/metrics/token_classification/metrics.py	98.79% <100.00%> (+0.07%)	⬆️
...rver/daos/backend/mappings/token_classification.py	100.00% <100.00%> (ø)
...ver/services/tasks/token_classification/metrics.py	94.01% <100.00%> (-5.24%)	⬇️

... and 2 files with indirect coverage changes

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