refactor `BatchHandler` into `ModelWorker`

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 73.99%. Comparing base (4019035) to head (7837c85).

Files	Patch %	Lines
...finity_emb/infinity_emb/inference/batch_handler.py	94.18%	5 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #202 +/- ## ========================================== + Coverage 73.71% 73.99% +0.27% ========================================== Files 32 32 Lines 1849 1884 +35 ========================================== + Hits 1363 1394 +31 - Misses 486 490 +4 ```

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