feat: adding stochastic positional embeddings

[kir-7]

Part of #1900

Summary

• Adds add_stochastic_positional_noise as utility function to lightly/models/utils.py .
• Adds optinal use of StoP to:
  * lightly/models/modules/ijepa.py and
  * ightly/models/modules/ijepa_timm.py
• Keeps StoP disabled by default to preserve existing behaviour.
• Adds a new test tests/utils/test_stochastic_positional_embedding.py to test the behaviour of StoP function while disabled and enabled.

Additional details:

• StoP can be used for masked image modelling (MIM) models and this PR uses it for IJEPA. It can be used for MAE as well, but currently MAEDecoder in lightly/models/modules does not provide functionality to modify the masks directly, these are modified while creating the MAE class (examples/pytorch/mae.py). These examples can be modified to evaluate StoP.

[gabrielfruet]

/review

[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.33%. Comparing base (9c0346f) to head (9c27f5f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lightly/models/modules/ijepa.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1963      +/-   ##
==========================================
- Coverage   86.36%   86.33%   -0.04%     
==========================================
  Files         172      172              
  Lines        7358     7371      +13     
==========================================
+ Hits         6355     6364       +9     
- Misses       1003     1007       +4     

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[kir-7]

@gabrielfruet The failing check seems to be regarding test coverage for ijepa.py for from_vit_encoder, should I add tests for this? And is there anything else I need to modify?