fix conditional sampling

Author: liuanji

src/pyjuice/queries/sample.py

@@ -307,7 +307,7 @@ def sample(pc: TensorCircuit, num_samples: Optional[int] = None, conditional: bo
     if not conditional:
         assert num_samples is not None, "`num_samples` should be specified when doing unconditioned sampling."
     else:
-        num_samples = pc.node_mars.size(0) # Reuse the batch size
+        num_samples = pc.node_mars.size(1) # Reuse the batch size
 
     root_ns = pc.root_ns
     assert root_ns._output_ind_range[1] - root_ns._output_ind_range[0] == 1, "It is ambiguous to sample from multi-head PCs."