prevent log10 L_op from upcasting

pytensor/scalar/basic.py

@@ -3030,7 +3030,7 @@ class Log10(UnaryScalarOp):
             else:
                 return [x.zeros_like()]
 
-        return (gz / (x * np.log(10.0)),)
+        return (gz / (x * np.asarray(math.log(10.0)).astype(x.dtype)),)
 
     def c_code(self, node, name, inputs, outputs, sub):
         (x,) = inputs

tests/scalar/test_basic.py

@@ -544,3 +544,14 @@ def test_shape():
     assert b.shape.type.ndim == 1
     assert b.shape.type.shape == (0,)
     assert b.shape.type.dtype == "int64"
+
+
+def test_grad_log10():
+    # Ensure that log10 does not upcast gradient
+    # This is a regression test for
+    # https://github.com/pymc-devs/pytensor/issues/667
+    a = float32("log10_a")
+    b = log10(a)
+    b_grad = pytensor.gradient.grad(b, a)
+    assert b.dtype == "float32"
+    assert b_grad.dtype == "float32"