fix unitary case in prior from trace

.gitignore

@@ -44,4 +44,4 @@ pytestdebug.log
 
 # Codespaces
 pythonenv*
-env/
+env/

pymc_experimental/tests/test_prior_from_trace.py

@@ -33,13 +33,21 @@ def coords():
     return dict(test=range(3), simplex=range(4))
 
 
-@pytest.fixture
-def user_param_cfg():
-    return ("t",), dict(
-        a="d",
-        b=dict(transform=transforms.log, dims=("test",)),
-        c=dict(transform=transforms.simplex, dims=("simplex",)),
-    )
+@pytest.fixture(
+    params=[
+        [
+            ("t",),
+            dict(
+                a="d",
+                b=dict(transform=transforms.log, dims=("test",)),
+                c=dict(transform=transforms.simplex, dims=("simplex",)),
+            ),
+        ],
+        [("t",), dict()],
+    ]
+)
+def user_param_cfg(request):
+    return request.param
 
 
 @pytest.fixture

pymc_experimental/utils/prior.py

@@ -94,6 +94,7 @@ def _flatten(idata: arviz.InferenceData, **kwargs: ParamCfg) -> FlatInfo:
 def _mean_chol(flat_array: np.ndarray):
     mean = flat_array.mean(0)
     cov = np.cov(flat_array, rowvar=False)
+    cov = np.atleast_2d(cov)
     chol = np.linalg.cholesky(cov)
     return mean, chol