[ci] fix: fix cpu_unit_test and tensordict CI on vllm_omni test, preparing for CI refactor

[ETOgaosion]

What does this PR do?

fix cpu_unit_test and tensordict CI on vllm_omni test, preparing for CI refactor

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[gemini-code-assist]

Code Review

This pull request enhances the handling of jagged NestedTensors by implementing custom utilities to bypass known PyTorch bugs in unbind and to_padded_tensor, specifically for 3D+ tensors and scenarios involving empty rows. It introduces _unbind_jagged_nested_tensor and _get_nested_tensor_ragged_idx to manage tensor splitting via values and offsets, and updates the multiturn_sft_dataset to support text-only tokenizers. Review feedback highlights potential performance regressions due to synchronous GPU-to-CPU data transfers when calling .tolist() on indices and .item() on tensor offsets within the new utility functions.