Fix scheduler/DDP/wandb reliability regressions and add tests

[erdoganxarda]

Summary

This PR fixes three reliability issues and adds regression tests.

Why

• Invalid --lr_policy handling returned a NotImplementedError object instead of raising.
• DDP norm validation logic/message was contradictory.
• wandb import failed even when --use_wandb was not enabled.

What Changed

• Raise NotImplementedError for unknown LR policies.
• In DDP mode, explicitly reject --norm batch and provide a clear error message.
• Make wandb import optional; only raise if --use_wandb is requested and wandb is missing.
• Added regression tests for all three cases.

Validation

• python -m compileall tests models util ✅
• Full pytest run was not executed in this shell (pytest/torch unavailable here).

Risk / Compatibility

• No intended behavior change for valid configs.
• DDP now fails fast with a clearer error for unsupported --norm batch.