Fix: PyTorch Example (#620)

Smaller syntax errors that prevented run.
Skip gracefully if PyTorch is not found.

Add more detail infos about slow startup (download).

examples/pytorch_surrogate_model/run_ml_surrogate_15_stage.py

@@ -16,7 +16,6 @@
     cupy_available = True
 except ImportError:
     cupy_available = False
-import sys
 
 import numpy as np
 import scipy.optimize as opt
@@ -36,7 +35,10 @@
     import torch
 except ImportError:
     print("Warning: Cannot import PyTorch. Skipping test.")
+    import sys
+
     sys.exit(0)
+
 import zipfile
 from urllib import request
 
@@ -91,6 +93,10 @@ def download_and_unzip(url, data_dir):
         zip_dataset.extractall()
 
 
+print(
+    "Downloading trained models from Zenodo.org - this might take a minute...",
+    flush=True,
+)
 data_url = "https://zenodo.org/records/10810754/files/models.zip?download=1"
 download_and_unzip(data_url, "models.zip")
 
@@ -299,8 +305,7 @@ def __init__(self, sim, stage_i, lattice_index, x_or_y):
         self.x_or_y = x_or_y
         self.push = self.set_lens
 
-    def set_lens(self, step):
-        pc = self.sim.particle_container()
+    def set_lens(self, pc, step):
         # get envelope parameters
         rbc = pc.reduced_beam_characteristics()
         alpha = rbc[f"alpha_{self.x_or_y}"]

examples/pytorch_surrogate_model/surrogate_model_definitions.py

@@ -8,8 +8,14 @@
 
 from enum import Enum
 
-import torch
-from torch import nn
+try:
+    import torch
+    from torch import nn
+except ImportError:
+    print("Warning: Cannot import PyTorch. Skipping test.")
+    import sys
+
+    sys.exit(0)
 
 
 class Activation(Enum):