fix: correct LoRALinear dtype and remove unused import

- LoRALinear now creates lora_A/lora_B with dtype matching the base
  linear's weight, preventing a float32/bf16 mismatch at forward time
  when the generator is loaded in bf16 or fp16.
- Remove unused `import math` from train_lora.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

train_lora.py

@@ -25,7 +25,6 @@ Usage:
 import argparse
 import os
 import sys
-import math
 import random
 import json
 from pathlib import Path