fix(sft): drop the cond cast deleted with loss_hub.utils

[Rockdu]

What

• Remove loss_hub/sft.py's import of the deleted loss_hub.utils.cast_cond_to_dtype and its cond pre-cast, mirroring what refactor(fsdp)(4.3/5): family-declared input dtype policy at the model boundary #103 did to flow_grpo/nft (whose leftover comment lines are dropped too).

Why

#90 (SFT) merged first and imports loss_hub.utils; #103 merged after and deleted it — both PRs were green, but every stage-a-cpu run on the merged main now fails at collection with ModuleNotFoundError. Boundary input dtypes are family input_dtype_policy territory since #103, and SFT batches flow through the same actor boundary; Wan declares no policy today, deliberately left as a separate family decision.

Checklist

• pre-commit run --all-files passes
• Added/updated tests for new behaviour — none needed: this restores collection of the existing test_loss_hub_sft.py
• pytest -x is green — not run locally (no local torch); stage-a-cpu re-runs the suite
• Remaining items — no flags/docs/examples touched