Refactor model export part 2: Add a dedicated forward function for model export

[vinnamkim]

Summary

• Ticket No. 138400
• Prior to this PR, there were hidden conditions where OTXModelExporter internally traced the OTXModel.model.forward function (not OTXModel.forward). This function needed to have a function signature such as Callable[[torch.Tensor], torch.Tensor | dict[str, torch.Tensor]] to correctly trace the model. Additionally, the auxiliary functions _reset_model_forward() and _restore_model_forward() were required to export an explainable model.
• However, this was not easily visible to a new model developer and was somewhat messy in design. Therefore, a dedicated forward function, OTXModel.forward_for_tracing(), has been explicitly added. Any derived model that wants to use OTXNativeModelExporter for its model export can implement this function.

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added e2e tests for validation.
• I have added the description of my changes into CHANGELOG in my target branch (e.g., CHANGELOG in develop).​
• I have updated the documentation in my target branch accordingly (e.g., documentation in develop).
• I have linked related issues.

License

• I submit my code changes under the same Apache License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2024 Intel Corporation
# SPDX-License-Identifier: Apache-2.0

[codecov]

Codecov Report

Attention: Patch coverage is 78.88199% with 34 lines in your changes are missing coverage. Please review.

Project coverage is 76.19%. Comparing base (c411844) to head (4b07554).
Report is 1 commits behind head on develop.

❗ Current head 4b07554 differs from pull request most recent head 6b7efc0. Consider uploading reports for the commit 6b7efc0 to get more accurate results

Files	Patch %	Lines
src/otx/core/model/base.py	80.76%	10 Missing ⚠️
src/otx/core/model/anomaly.py	20.00%	8 Missing ⚠️
src/otx/engine/engine.py	84.21%	6 Missing ⚠️
src/otx/algo/classification/torchvision_model.py	40.00%	3 Missing ⚠️
src/otx/core/model/classification.py	50.00%	3 Missing ⚠️
src/otx/core/model/utils/mmpretrain.py	90.47%	2 Missing ⚠️
src/otx/core/model/action_classification.py	50.00%	1 Missing ⚠️
src/otx/core/model/segmentation.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3317      +/-   ##
===========================================
- Coverage    77.15%   76.19%   -0.97%     
===========================================
  Files          204      194      -10     
  Lines        17526    16688     -838     
===========================================
- Hits         13523    12715     -808     
+ Misses        4003     3973      -30     

Flag	Coverage Δ
py310	?
py311	76.19% <78.88%> (-0.97%)	⬇️

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