Revert "adding OnlineSampleMapping"
jenkins Is there a CI test which uses context parallel? @pablo-garay @ericharper 
cc @michalivne  jenkins @xrennvidia yes but w/o CI its hard to debug/iterate on the PR.
 jenkins
Reverts NVIDIA/NeMo#8137

Nemo pipeline is broken (finetune tests failing) due to this. Hence revert.

More context/details:
- two gpt3 fine-tunning tests failed on EOS (finetune.gpt3.126m_tp1_pp1_1node_100steps_squad and finetune.gpt3.400m_improved_tp1_pp1_1node_100steps_squad). Check out the error log, it’s not the fault of context parallelism. The error comes from dataset, which is introduced by this PR https://github.com/NVIDIA/NeMo/pull/8137

[10:43](https://nvidia.slack.com/archives/D062J0U5PJ8/p1705084988269489)

Above problematic PR was merged after my context parallelism PR. So for sanity check, I also ran a pipeline with the NeMo commit ID of context parallelism merging (i.e., without above problematic PR) 
you can see those two fine-tuning test passed, so I think context parallelism code works fine