Move data_samplers.py from legacy to training.datasets & add DistributedSignalHandler to DataLoader workers

[asolergi-nv]

What does this PR do ?

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share discuss a design-doc with the team.

Move data_samplers.py from legacy to core.datasets & add DistributedSignalHandler to DataLoader workers. Now the user can also select other signals, not only SIGTERM. This will help us ensure that we will successfully and gracefully exit the job. In some setups, the DataLoader workers are kept alive, so the job doesn’t finish until it hits the job wall time limit.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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[asolergi-nv]

/ok to test ebfbb1e