Fix broadcasting bug in vectorization of RandomVariables

pytensor/tensor/random/op.py

@@ -20,6 +20,7 @@
 )
 from pytensor.tensor.random.type import RandomGeneratorType, RandomStateType, RandomType
 from pytensor.tensor.random.utils import (
+    compute_batch_shape,
     explicit_expand_dims,
     normalize_size_param,
 )
@@ -403,15 +404,14 @@ def vectorize_random_variable(
         original_expanded_dist_params, dict(zip(original_dist_params, dist_params))
     )
 
-    if len_old_size and equal_computations([old_size], [size]):
+    new_ndim = dist_params[0].type.ndim - original_expanded_dist_params[0].type.ndim
+
+    if new_ndim and len_old_size and equal_computations([old_size], [size]):
         # If the original RV had a size variable and a new one has not been provided,
         # we need to define a new size as the concatenation of the original size dimensions
         # and the novel ones implied by new broadcasted batched parameters dimensions.
-        # We use the first broadcasted batch dimension for reference.
-        bcasted_param = explicit_expand_dims(dist_params, op.ndims_params)[0]
-        new_param_ndim = (bcasted_param.type.ndim - op.ndims_params[0]) - len_old_size
-        if new_param_ndim >= 0:
-            new_size_dims = bcasted_param.shape[:new_param_ndim]
-            size = concatenate([new_size_dims, size])
+        broadcasted_batch_shape = compute_batch_shape(dist_params, op.ndims_params)
+        new_size_dims = broadcasted_batch_shape[:new_ndim]
+        size = concatenate([new_size_dims, size])
 
     return op.make_node(rng, size, dtype, *dist_params)

pytensor/tensor/random/utils.py

@@ -11,7 +11,7 @@
 from pytensor.scalar import ScalarVariable
 from pytensor.tensor import get_vector_length
 from pytensor.tensor.basic import as_tensor_variable, cast, constant
-from pytensor.tensor.extra_ops import broadcast_to
+from pytensor.tensor.extra_ops import broadcast_arrays, broadcast_to
 from pytensor.tensor.math import maximum
 from pytensor.tensor.shape import shape_padleft, specify_shape
 from pytensor.tensor.type import int_dtypes
@@ -149,6 +149,15 @@ def explicit_expand_dims(
     return new_params
 
 
+def compute_batch_shape(params, ndims_params: Sequence[int]) -> TensorVariable:
+    params = explicit_expand_dims(params, ndims_params)
+    batch_params = [
+        param[(..., *(0,) * core_ndim)]
+        for param, core_ndim in zip(params, ndims_params)
+    ]
+    return broadcast_arrays(*batch_params)[0].shape
+
+
 def normalize_size_param(
     size: int | np.ndarray | Variable | Sequence | None,
 ) -> Variable:

tests/tensor/random/test_op.py

@@ -292,6 +292,14 @@ def test_vectorize_node():
     assert vect_node.op is normal
     assert vect_node.default_output().type.shape == (10, 5)
 
+    node = normal(vec, size=(5,)).owner
+    new_inputs = node.inputs.copy()
+    new_inputs[3] = tensor("mu", shape=(1, 5))  # mu
+    new_inputs[4] = tensor("sigma", shape=(10,))  # sigma
+    vect_node = vectorize_node(node, *new_inputs)
+    assert vect_node.op is normal
+    assert vect_node.default_output().type.shape == (10, 5)
+
     # Test parameter broadcasting with expanding size
     node = normal(vec, size=(2, 5)).owner
     new_inputs = node.inputs.copy()