gracefully handle when pytorch is not installed

pygsti/forwardsims/torchfwdsim.py

@@ -173,11 +173,14 @@ def jac_friendly_circuit_probs(self, *free_params: Tuple[torch.Tensor]):
 
 
 class TorchForwardSimulator(ForwardSimulator):
+
+    ENABLED = TORCH_ENABLED
+
     """
     A forward simulator that leverages automatic differentiation in PyTorch.
     """
     def __init__(self, model : Optional[ExplicitOpModel] = None):
-        if not TORCH_ENABLED:
+        if not TorchForwardSimulator.ENABLED:
             raise RuntimeError('PyTorch could not be imported.')
         self.model = model
         super(ForwardSimulator, self).__init__(model)

setup.py

@@ -35,6 +35,7 @@
 
 # Extra requirements
 extras = {
+    'pytorch' : ['torch'],
     'diamond_norm': [
         'cvxopt',
         'cvxpy'

test/unit/objects/test_forwardsim.py

@@ -3,6 +3,7 @@
 from unittest import mock
 
 import numpy as np
+import pytest
 
 from pygsti.models import modelconstruction as _setc
 import pygsti.models as models
@@ -177,6 +178,7 @@ def test_simple_matrix_fwdsim(self):
     def test_simple_map_fwdsim(self):
         self._run(SimpleMapForwardSimulator)
 
+    @pytest.mark.skipif(not TorchForwardSimulator.ENABLED, reason="PyTorch is not installed.")
     def test_torch_fwdsim(self):
         self._run(TorchForwardSimulator)