Use the generate config sampler instead of the undefined train_config in GenerateProcess

The sampler-building branch in GenerateProcess.__init__ referenced
`self.train_config.noise_scheduler`, but GenerateProcess never defines
`self.train_config` (only training processes do). Running a `generate`
job that hits this branch crashes at construction:

    AttributeError: 'GenerateProcess' object has no attribute 'train_config'

Use the sampler from `self.generate_config`, which is already built
earlier in the same __init__. This branch currently always crashes, so
no working path is affected.

jobs/process/GenerateProcess.py

@@ -102,7 +102,7 @@ class GenerateProcess(BaseProcess):
             if self.model_config.is_lumina2:
                 arch = 'lumina2'
             sampler = get_sampler(
-                self.train_config.noise_scheduler,
+                self.generate_config.sampler,
                 {
                     "prediction_type": "v_prediction" if self.model_config.is_v_pred else "epsilon",
                 },