Add repository fallback for experimental checkpoint downloads

[recrack]

Fixes: #197

Problem

Users may lack permission to nvidia/Cosmos-Experimental but have access to nvidia-cosmos-ea/Cosmos-Experimental. Current implementation fails immediately without trying the alternative repository.

Summary

Add automatic repository fallback for experimental checkpoints. When nvidia/Cosmos-Experimental download fails, automatically retry with nvidia-cosmos-ea/Cosmos-Experimental.

Changes

cosmos_transfer2/_src/imaginaire/utils/checkpoint_db.py

• Added repository list logic in CheckpointFileHf._download()
• Implemented try-except loop to retry with fallback repository on failure
• Added informative RuntimeError when all repositories fail

cosmos_transfer2/_src/imaginaire/utils/checkpoint_db_test.py

• test_experimental_repository_fallback: Verifies successful fallback from primary to alternative repository
• test_non_experimental_repository_no_fallback: Ensures non-experimental repositories don't use fallback behavior
• test_both_repositories_fail: Confirms RuntimeError is raised when all repositories fail

Test Results: All 3 tests pass successfully:

cosmos_transfer2/_src/imaginaire/utils/checkpoint_db_test.py::test_experimental_repository_fallback PASSED [ 33%]
cosmos_transfer2/_src/imaginaire/utils/checkpoint_db_test.py::test_non_experimental_repository_no_fallback PASSED [ 66%]
cosmos_transfer2/_src/imaginaire/utils/checkpoint_db_test.py::test_both_repositories_fail PASSED [100%]

3 passed, 7 warnings in 0.83s

[recrack]

'll run lint checks according to the contributing guidelines and submit a fix commit

[recrack]

Updated. test result.

---

just lint
uv tool install "pre-commit>=4.3.0"
`pre-commit>=4.3.0` is already installed
pre-commit install -c .pre-commit-config-base.yaml
pre-commit installed at .git/hooks/pre-commit
pre-commit run -a  || pre-commit run -a
addlicense...............................................................Passed
markdown-toc-creator.....................................................Passed
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check for broken symlinks............................(no files to check)Skipped
Generate uv lock files for projects......................................Passed
Generate uv lock files for scripts.......................................Passed
ruff fix.................................................................Passed
ruff format..............................................................Passed
link check relative......................................................Passed

$ just test
...
tests/assets_test.py::test_inference_assets[car_example/multicontrol]
tests/assets_test.py::test_inference_assets[image_example]
tests/assets_test.py::test_inference_assets[robot_example/edge]
tests/assets_test.py::test_inference_assets[robot_example/vis0]
tests/assets_test.py::test_inference_assets[robot_example/depth]
tests/assets_test.py::test_inference_assets[car_example/edge]
tests/assets_test.py::test_inference_assets[car_example/seg]
tests/assets_test.py::test_inference_assets[multiview_example]
tests/assets_test.py::test_inference_assets[robot_example/vis1]
tests/assets_test.py::test_inference_assets[robot_multiview_control-agibot]
[gw3] [  8%] PASSED tests/assets_test.py::test_inference_assets[image_example]
[gw0] [ 16%] PASSED tests/assets_test.py::test_inference_assets[car_example/edge]
[gw6] [ 25%] PASSED tests/assets_test.py::test_inference_assets[robot_example/vis1]
[gw1] [ 33%] PASSED tests/assets_test.py::test_inference_assets[car_example/seg]
[gw5] [ 41%] PASSED tests/assets_test.py::test_inference_assets[robot_example/edge]
[gw2] [ 50%] PASSED tests/assets_test.py::test_inference_assets[car_example/multicontrol]
[gw4] [ 58%] PASSED tests/assets_test.py::test_inference_assets[robot_example/depth]
[gw9] [ 66%] PASSED tests/assets_test.py::test_inference_assets[multiview_example]
tests/assets_test.py::test_inference_assets[robot_example/multicontrol]
[gw10] [ 75%] PASSED tests/assets_test.py::test_inference_assets[robot_example/multicontrol]
[gw13] [ 83%] PASSED tests/assets_test.py::test_inference_assets[robot_multiview_control-agibot]
tests/assets_test.py::test_inference_assets[robot_example/seg]
[gw8] [ 91%] PASSED tests/assets_test.py::test_inference_assets[robot_example/seg]
[gw7] [100%] PASSED tests/assets_test.py::test_inference_assets[robot_example/vis0]

============================================================================================= slowest 5 durations ==============================================================================================
0.00s call     tests/assets_test.py::test_inference_assets[multiview_example]
0.00s call     tests/assets_test.py::test_inference_assets[robot_multiview_control-agibot]
0.00s call     tests/assets_test.py::test_inference_assets[car_example/seg]
0.00s call     tests/assets_test.py::test_inference_assets[robot_example/multicontrol]
0.00s call     tests/assets_test.py::test_inference_assets[car_example/multicontrol]
============================================================================================== 12 passed in 4.90s ==============================================================================================
+