Make sure new size values are int64

Closes #4652.

pymc3/aesaraf.py

@@ -156,6 +156,10 @@ def change_rv_size(
             size = rv_node.op._infer_shape(size, dist_params)
         new_size = tuple(np.atleast_1d(new_size)) + tuple(size)
 
+    # Make sure the new size is int64 so that it doesn't unnecessarily pick
+    # up a `Cast` in some cases
+    new_size = at.as_tensor(new_size, ndim=1, dtype="int64")
+
     new_rv_node = rv_node.op.make_node(rng, new_size, dtype, *dist_params)
     rv_var = new_rv_node.outputs[-1]
     rv_var.name = name

pymc3/tests/test_aesaraf.py

@@ -23,7 +23,7 @@
 import pytest
 import scipy.sparse as sps
 
-from aesara.graph.basic import Variable, ancestors
+from aesara.graph.basic import Constant, Variable, ancestors
 from aesara.tensor.random.basic import normal, uniform
 from aesara.tensor.random.op import RandomVariable
 from aesara.tensor.subtensor import AdvancedIncSubtensor, AdvancedIncSubtensor1
@@ -67,6 +67,15 @@ def test_change_rv_size():
     assert rv_newer.ndim == 3
     assert rv_newer.eval().shape == (4, 3, 2)
 
+    # Make sure we avoid introducing a `Cast` by converting the new size before
+    # constructing the new `RandomVariable`
+    rv = normal(0, 1)
+    new_size = np.array([4, 3], dtype="int32")
+    rv_newer = change_rv_size(rv, new_size=new_size, expand=False)
+    assert rv_newer.ndim == 2
+    assert isinstance(rv_newer.owner.inputs[1], Constant)
+    assert rv_newer.eval().shape == (4, 3)
+
 
 class TestBroadcasting:
     def test_make_shared_replacements(self):