build_backgrounds_chrombpnet: --gpu N overrides outer CUDA_VISIBLE_DEVICES, breaking parallel-launch pattern

[lucapinello]

Background

Caught during the ChromBPNet CDF rebuild on 2026-04-30 (audits/2026-04-29_chrombpnet_cdf_rebuild/report.md, F2).

scripts/build_backgrounds_chrombpnet.py:200 does:

```python
os.environ["CUDA_VISIBLE_DEVICES"] = str(args.gpu)
```

…which overrides any pre-set CUDA_VISIBLE_DEVICES. The HANDOFF.md's parallel-launch suggestion uses the standard pattern:

```bash

Terminal 1: GPU 0

CUDA_VISIBLE_DEVICES=0 ... --gpu 0 ...

Terminal 2: GPU 1

CUDA_VISIBLE_DEVICES=1 ... --gpu 0 ... # outer var pins to GPU 1, --gpu 0 is "first visible"
```

But because the script clobbers the outer var, the second job lands on physical GPU 0 too, fighting the first job for memory and OOMing.

Suggested fix

Honour a pre-set CUDA_VISIBLE_DEVICES if present; only set it from --gpu N when no env var was provided:

```python
if "CUDA_VISIBLE_DEVICES" not in os.environ:
os.environ["CUDA_VISIBLE_DEVICES"] = str(args.gpu)
```

Related

• chorus PR prep: ChromBPNet CDF rebuild against chrombpnet_nobias (script + handoff) #67 (CDF rebuild prep)
• audit at audits/2026-04-29_chrombpnet_cdf_rebuild/report.md (F2 finding)

[lucapinello]

Fixed in #76 (merged as 45c8d91). Duplicate of #72; fix is the conditional CUDA_VISIBLE_DEVICES guard in scripts/build_backgrounds_chrombpnet.py:200.