ci: test_fused_indexer_loss_gradient_tp_consistency is flaky in dev

## Description

`test_fused_indexer_loss_gradient_tp_consistency` in `tests/unit_tests/transformer/experimental_attention_variant/test_attention_variant_dsa.py` is flaky in the dev container CI.

## Failure

- CI run: https://github.com/NVIDIA/Megatron-LM/actions/runs/24403010080/job/71283603051#step:3:998

## Fix

The test has been marked with `@pytest.mark.flaky_in_dev` in:
- PR: #4299

## Owner

Test introduced by @kunlunl.