Fix numba impl of empty DimShuffle (#218)

Empty DimShuffles would return a scalar instead of an array, which can then lead to errors in ops that expect an array.

Author: aseyboldt

pytensor/link/numba/dispatch/elemwise.py

@@ -841,7 +841,7 @@ def dimshuffle_inner(x, shuffle):
 
         @numba_basic.numba_njit
         def dimshuffle_inner(x, shuffle):
-            return x.item()
+            return np.reshape(x, ())
 
     # Without the following wrapper function we would see this error:
     # E   No implementation of function Function(<built-in function getitem>) found for signature:

tests/link/numba/test_elemwise.py

@@ -210,6 +210,14 @@ def test_Dimshuffle(v, new_order):
     )
 
 
+def test_Dimshuffle_returns_array():
+    x = at.vector("x", shape=(1,))
+    y = 2 * at_elemwise.DimShuffle([True], [])(x)
+    func = pytensor.function([x], y, mode="NUMBA")
+    out = func(np.zeros(1, dtype=config.floatX))
+    assert out.ndim == 0
+
+
 @pytest.mark.parametrize(
     "careduce_fn, axis, v",
     [