Fix SoftmaxGrad failure with constant dy in numba backend

pytensor/link/numba/dispatch/elemwise.py

@@ -925,7 +925,12 @@ def numba_funcify_SoftmaxGrad(op, node, **kwargs):
         dx = dy_times_sm - sum_dy_times_sm * sm
         return dx
 
-    softmax_grad = jit_compile_reducer(node, softmax_grad_py_fn)
+    # The signature inferred by jit_compile_reducer is wrong when dy is a constant (readonly=True)
+    # softmax_grad = jit_compile_reducer(node, softmax_grad_py_fn)
+    softmax_grad = numba_njit(
+        boundscheck=False,
+        fastmath=config.numba__fastmath,
+    )(softmax_grad_py_fn)
 
     return softmax_grad
 

tests/link/numba/test_elemwise.py

@@ -445,6 +445,16 @@ def test_SoftmaxGrad(dy, sm, axis, exc):
         )
 
 
+def test_SoftMaxGrad_constant_dy():
+    dy = at.constant(np.zeros((3,), dtype=config.floatX))
+    sm = at.vector(shape=(3,))
+
+    g = SoftmaxGrad(axis=None)(dy, sm)
+    g_fg = FunctionGraph(outputs=[g])
+
+    compare_numba_and_py(g_fg, [np.ones((3,), dtype=config.floatX)])
+
+
 @pytest.mark.parametrize(
     "x, axis, exc",
     [