RHO-LOSS Step 4: Implement `RHOLossRemoteDownsampler`

[XianzheMa]

This PR implements the remote RHO-LOSS downsampler at trainer server, thus finishing the implementation of RHO-LOSS.

How to review

The number of touched files is huge but most of them are nominal changes. Please first take a look at remote_rho_loss_downsampling.py, then at irreducible_loss_producer.py, then at their corresponding unit test files, then at rest files.

What's left

I still need an integration test to connect all pieces together as this selection policy is non-trivial.

[github-actions[bot]]

^{( % to main) ( % to main)}

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.85039% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.03%. Comparing base (4373fe2) to head (3b91ef5).

Files	Patch %	Lines
...trainer_server/internal/trainer/pytorch_trainer.py	33.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #491 +/- ## ========================================== - Coverage 83.90% 82.03% -1.88% ========================================== Files 209 214 +5 Lines 9633 9948 +315 ========================================== + Hits 8083 8161 +78 - Misses 1550 1787 +237 ```

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[MaxiBoether]

feel free to reduce the required coverage to 82%, if that's the blocking issue here