Integrate MoE as first-class component

[rrutmann]

What does this PR do?

This PR migrates MoE support from the side package under moe into the main modalities codebase, integrating it as first-class registry components with FSDP2 and Expert Parallel (EP) support. It also adds a new MoE training config set and a focused MoE unit test suite.

General Changes

• Core MoE integration
• Use shared core for rotary embeddings for qwen and GPT2 model
• Added configs for FSDP and EP MoE training
• Runtime fixes:
  • Mixed precision dtype mismatch fix in expert matmul path (BF16 activations vs FP32 expert params)
  • Checkpoint optimizer-state compatibility fix for custom EP optimizer state schemas
• Added tests

Breaking Changes

• ..

Checklist before submitting final PR

• My PR is minimal and addresses one issue in isolation
• I have merged the latest version of the target branch into this feature branch
• I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• I have run a sample config for model training
• I have checked that all tests run through (python tests/tests.py)
• I have updated the internal changelog (CHANGELOG_DEV.md)