Suggest var_names when using deprecated API for partial traces (#7289)

* Update docs when list is passed to trace in sample for partial trace

* Change DeprecationWarning to ValueError

Co-authored-by: Ricardo Vieira <28983449+ricardoV94@users.noreply.github.com>

* Update test_partial_trace_unsupported

---------

Co-authored-by: Ricardo Vieira <28983449+ricardoV94@users.noreply.github.com>

Author: lancechua

pymc/sampling/mcmc.py

@@ -630,10 +630,7 @@ def sample(
         else:
             kwargs["nuts"] = {"target_accept": kwargs.pop("target_accept")}
     if isinstance(trace, list):
-        raise DeprecationWarning(
-            "We have removed support for partial traces because it simplified things."
-            " Please open an issue if & why this is a problem for you."
-        )
+        raise ValueError("Please use `var_names` keyword argument for partial traces.")
 
     model = modelcontext(model)
     if not model.free_RVs:

tests/sampling/test_mcmc.py

@@ -507,11 +507,11 @@ def test_empty_model():
         error.match("any free variables")
 
 
-def test_partial_trace_unsupported():
+def test_partial_trace_with_trace_unsupported():
     with pm.Model() as model:
         a = pm.Normal("a", mu=0, sigma=1)
         b = pm.Normal("b", mu=0, sigma=1)
-        with pytest.raises(DeprecationWarning, match="removed support"):
+        with pytest.raises(ValueError, match="var_names"):
             pm.sample(trace=[a])