only treat () as scalar shapes

closes pymc-devs#4206

pymc3/distributions/distribution.py

@@ -943,11 +943,9 @@ def _draw_value(param, point=None, givens=None, size=None):
 
 
 def _is_one_d(dist_shape):
-    if hasattr(dist_shape, "dshape") and dist_shape.dshape in ((), (0,), (1,)):
+    if hasattr(dist_shape, "dshape") and dist_shape.dshape in { (), }:
         return True
-    elif hasattr(dist_shape, "shape") and dist_shape.shape in ((), (0,), (1,)):
-        return True
-    elif to_tuple(dist_shape) == ():
+    elif hasattr(dist_shape, "shape") and dist_shape.shape in { (), }:
         return True
     return False
 
@@ -1069,6 +1067,7 @@ def generate_samples(generator, *args, **kwargs):
             len(samples.shape) > len(dist_shape)
             and samples.shape[-len(dist_shape) :] == dist_shape[-len(dist_shape) :]
         ):
+            raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
             samples = samples.reshape(samples.shape[1:])
 
     if (
@@ -1077,5 +1076,6 @@ def generate_samples(generator, *args, **kwargs):
         and samples.shape[-1] == 1
         and (samples.shape != size_tup or size_tup == tuple() or size_tup == (1,))
     ):
+        raise ValueError(f"This SHOULD be unreachable code. DON'T MERGE UNTIL THIS ENTIRE BLOCK WAS REMOVED. {samples.shape}, {size_tup}")
         samples = samples.reshape(samples.shape[:-1])
     return np.asarray(samples)