PREVENT 1D DISTRIBUTION (#633)

* PREVENT 1D DISTRIBUTION

* PREVENT 1D DISTRIBUTION

blackjax/mcmc/mclmc.py

@@ -22,7 +22,7 @@
 from blackjax.base import SamplingAlgorithm
 from blackjax.mcmc.integrators import IntegratorState, isokinetic_mclachlan
 from blackjax.types import ArrayLike, PRNGKey
-from blackjax.util import generate_unit_vector
+from blackjax.util import generate_unit_vector, pytree_size
 
 __all__ = ["MCLMCInfo", "init", "build_kernel", "mclmc"]
 
@@ -45,6 +45,10 @@ class MCLMCInfo(NamedTuple):
 
 
 def init(position: ArrayLike, logdensity_fn, rng_key):
+    if pytree_size(position) < 2:
+        raise ValueError(
+            "The target distribution must have more than 1 dimension for MCLMC."
+        )
     l, g = jax.value_and_grad(logdensity_fn)(position)
 
     return IntegratorState(