feat(mimo): phase 3 - data loading

[aroshanghias-nvd]

What does this PR do ?

Add Phase 3 MIMO data loading scaffolding with config, mock datasets, and loader integration that respects per-module DP and PP gating.

Changelog

• Add MIMODataConfig with encoder metadata resolution/validation and per‑module micro‑batch sizing.
• Add mock MIMO dataset, collate function, and provider with dynamic encoder support.
• Add MIMO dataset builder and integrate MIMO path into data loaders and iterators.
• Add unit tests for collate shapes, DP/PP gating, and MIMO data config validation.

GitHub Actions CI

See the CI sectionin the Contributing doc for how to trigger the CI. A Nvidia developer will need to approve and trigger the CI for external contributors.

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests?
• Did you add or update any necessary documentation?
• Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  • Reviewer: Does the PR have correct import guards for all optional libraries?

If you haven't finished some of the above items you can still open "Draft" PR.

Additional Information

• Related to # (issue)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.