Multi-GPU training sample generation problem - FLUX

One of my follower had below error while generating samples during 2x GPU training @kohya-ss

```
Encoding prompt: Style of EC$, a heart shaped character with pink arms and legs, long eyes, and small pink lips. The character is making a V peace sign with one oh his hands. The character is wearing black boots. The background is light blue.
[torch.Size([1, 768]), None, None, None]
0%| | 0/25 [00:00
[rank1]: trainer.train(args)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/train_network.py", line 1253, in train
[rank1]: self.sample_images(accelerator, args, epoch + 1, global_step, accelerator.device, vae, tokenizers, text_encoder, unet)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/flux_train_network.py", line 291, in sample_images
[rank1]: flux_train_utils.sample_images(
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/library/flux_train_utils.py", line 113, in sample_images
[rank1]: sample_image_inference(
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/library/flux_train_utils.py", line 229, in sample_image_inference
[rank1]: x = denoise(flux, noise, img_ids, t5_out, txt_ids, l_pooled, timesteps=timesteps, guidance=scale, t5_attn_mask=t5_attn_mask)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/library/flux_train_utils.py", line 314, in denoise
[rank1]: pred = model(
[rank1]: File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1553, in _wrapped_call_impl
[rank1]: return self._call_impl(*args, **kwargs)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1562, in _call_impl
[rank1]: return forward_call(*args, **kwargs)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/accelerate/utils/operations.py", line 819, in forward
[rank1]: return model_forward(*args, **kwargs)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/accelerate/utils/operations.py", line 807, in __call__
[rank1]: return convert_to_fp32(self.model_forward(*args, **kwargs))
[rank1]: File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/amp/autocast_mode.py", line 43, in decorate_autocast
[rank1]: return func(*args, **kwargs)
[rank1]: File "/home/Ubuntu/apps/kohya_ss/sd-scripts/library/flux_models.py", line 1004, in forward
[rank1]: if img.ndim != 3 or txt.ndim != 3:
[rank1]: AttributeError: 'NoneType' object has no attribute 'ndim'
W0908 17:26:18.982000 132414536454144 torch/distributed/elastic/multiprocessing/api.py:858] Sending process 12066 closing signal SIGTERM
E0908 17:26:20.101000 132414536454144 torch/distributed/elastic/multiprocessing/api.py:833] failed (exitcode: 1) local_rank: 1 (pid: 12067) of binary: /home/Ubuntu/apps/kohya_ss/venv/bin/python
Traceback (most recent call last):
File "/home/Ubuntu/apps/kohya_ss/venv/bin/accelerate", line 8, in
sys.exit(main())
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/accelerate/commands/accelerate_cli.py", line 48, in main
args.func(args)
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/accelerate/commands/launch.py", line 1097, in launch_command
multi_gpu_launcher(args)
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/accelerate/commands/launch.py", line 734, in multi_gpu_launcher
distrib_run.run(args)
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/distributed/run.py", line 892, in run
elastic_launch(
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/distributed/launcher/api.py", line 133, in __call__
return launch_agent(self._config, self._entrypoint, list(args))
File "/home/Ubuntu/apps/kohya_ss/venv/lib/python3.10/site-packages/torch/distributed/launcher/api.py", line 264, in launch_agent
raise ChildFailedError(
torch.distributed.elastic.multiprocessing.errors.ChildFailedError:
============================================================
/home/Ubuntu/apps/kohya_ss/sd-scripts/flux_train_network.py FAILED
------------------------------------------------------------
Failures:

------------------------------------------------------------
Root Cause (first observed failure):
[0]:
time : 2024-09-08_17:26:18
host : 0053-kci-prxmx10033
rank : 1 (local_rank: 1)
exitcode : 1 (pid: 12067)
error_file:
traceback : To enable traceback see: https://pytorch.org/docs/stable/elastic/errors.html
============================================================
17:26:21-518520 INFO Training has ended.
^[c^[z

```

Start with flux_train_network.py:291 and follow sample generation into library/flux_train_utils.py:113, 229, and 314, then inspect the forward call at library/flux_models.py:1004. Reproduce the sample-generation path with two GPUs and determine why a None value reaches the model; done means multi-GPU training generates samples without the reported AttributeError.

Written by the indexing model from the issue text.