Do not install nutpie for external sampler tests

Conda nutpie, and soon pip, depend on the last release of PyMC and by association PyTensor, leading to dependency issues when trying to test compatibility after PyMC bumps the PyTensor dependency.

.github/workflows/tests.yml

@@ -363,7 +363,6 @@ jobs:
       - name: Install external samplers
         run: |
           conda activate pymc-test
-          conda install -c conda-forge nutpie
           pip install "numpyro>=0.8.0"
           pip install git+https://github.com/blackjax-devs/blackjax.git@0.7.0
       - name: Run tests

tests/sampling/test_mcmc_external.py

@@ -17,16 +17,15 @@
 
 from pymc import Model, Normal, sample
 
-pytest.importorskip("nutpie")
-pytest.importorskip("blackjax")
-pytest.importorskip("numpyro")
-
 # turns all warnings into errors for this module
 pytestmark = pytest.mark.filterwarnings("error")
 
 
 @pytest.mark.parametrize("nuts_sampler", ["pymc", "nutpie", "blackjax", "numpyro"])
 def test_external_nuts_sampler(recwarn, nuts_sampler):
+    if nuts_sampler != "pymc":
+        pytest.importorskip(nuts_sampler)
+
     with Model():
         Normal("x")