[DCNV2] Add MLPerf logging

[janekl]

Author: Jan Lasek (jlasek_at_nvidia.com)

This PR adds MLPerf logging to the new recommendation benchmark - DCNV2 model.

• reviewed by @mmarcinkiewicz and @samiwilf
• this is equivalent of https://github.com/facebookresearch/dlrm/pull/329 (original sources for https://github.com/mlcommons/training/pull/612)
• logs produced are MLPerf-compliant (a follow-up PR to https://github.com/mlcommons/logging coming soon)

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[erichan1]

@janekl you might need to reopen this PR after you sign CLA. Let me know if you have issues.

[janekl]

OK, let me close this PR and reopen it