Prior predictions constant data (#5723)

* support return constant_data with prior pred

* pre-commit format

* added test

* code review

Author: TimOliverMaier

pymc/backends/arviz.py

@@ -464,7 +464,6 @@ def observed_data_to_xarray(self):
             default_dims=[],
         )
 
-    @requires(["trace", "predictions"])
     @requires("model")
     def constant_data_to_xarray(self):
         """Convert constant data to xarray."""

pymc/tests/test_idata_conversion.py

@@ -502,7 +502,7 @@ def test_no_trace(self):
 
     @pytest.mark.parametrize("use_context", [True, False])
     def test_priors_separation(self, use_context):
-        """Test model is enough to get prior, prior predictive and observed_data."""
+        """Test model is enough to get prior, prior predictive, constant_data and observed_data."""
         with pm.Model() as model:
             x = pm.MutableData("x", [1.0, 2.0, 3.0])
             y = pm.ConstantData("y", [1.0, 2.0, 3.0])
@@ -514,6 +514,7 @@ def test_priors_separation(self, use_context):
             "prior": ["beta", "~obs"],
             "observed_data": ["obs"],
             "prior_predictive": ["obs"],
+            "constant_data": ["x", "y"],
         }
         if use_context:
             with model: