Remove BasePlugin

[four4fish]

What does this PR do?

deprecate unnecessary interface, flatten plugins

Next step : Unify TrainingTypePlugin and PrecisionPlugin interface

Tests:
python -m pytest -v tests/trainer. passed
python -m pytest -v tests/plugins passed

Fixes #8988

Does your PR introduce any breaking changes? If yes, please list them.

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[codecov]

Codecov Report

Merging #9066 (c5174ad) into master (de57fef) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9066    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         176     175     -1     
  Lines       14663   14665     +2     
=======================================
- Hits        13496   12888   -608     
- Misses       1167    1777   +610     

[awaelchli]

I think we can remove it directly.

[awaelchli]

@four4fish do you have any details on

Next step : Unify TrainingTypePlugin and PrecisionPlugin interface

Let's discuss on a GitHub issue?

[ananthsub]

Let's discuss on a GitHub issue?

+1 - let's use this issue to discuss the new proposal: #7324