Fix Cosmos2 safety checker grad mode leak

[ooovenenoso]

What does this PR do?

Fixes #13790.

Cosmos2VideoToWorldPipeline creates a default CosmosSafetyChecker when safety_checker=None. If the guardrail dependency changes PyTorch's global grad mode during construction, that state leaks out of from_pretrained() and leaves torch.is_grad_enabled() disabled globally.

This preserves and restores the caller's grad mode around default safety checker construction.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@DN6

Tests

• Added test_default_safety_checker_preserves_grad_mode to cover default safety checker construction that disables grad mode.
• Verified RED locally by running the new test before the fix and observing it fail with AssertionError: False is not true.
• python -m pytest tests/pipelines/cosmos/test_cosmos2_video2world.py::Cosmos2VideoToWorldPipelineFastTests::test_default_safety_checker_preserves_grad_mode -q
• python -m pytest tests/pipelines/cosmos/test_cosmos2_video2world.py -q
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