Fix unsynced warmup across selector instances

[XianzheMa]

This PR acts more as a patch rather than a fundamental fix to the asynchronism across different selector instances.

A fundamental fix should make the selection strategy stateless: by getting rid of the fieldself._next_trigger_id and providing it in the upstream as an argument.

This PR reduces the statefullness by removing the self.is_warmup.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.14%. Comparing base (5ad108a) to head (b768242).

Files	Patch %	Lines
...r/internal/selector_strategies/coreset_strategy.py	50.00%	4 Missing :warning:
modyn/selector/selector.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #512 +/- ## ========================================== - Coverage 82.18% 82.14% -0.04% ========================================== Files 214 214 Lines 9961 9962 +1 ========================================== - Hits 8186 8183 -3 - Misses 1775 1779 +4 ```

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