modify slice op Infershape (#45855)

* modify slice infershape

* code style

* modify slice_unittest

paddle/phi/kernels/funcs/slice_utils.h

@@ -117,6 +117,10 @@ inline phi::DDim GetSliceDims(const phi::DDim in_dims,
       continue;
     }
 
+    if (in_dims[axis] == -1) {
+      continue;
+    }
+
     T start = starts[i];
     T end = ends[i];
     T step = steps == nullptr ? 1 : (*steps)[i];

python/paddle/fluid/tests/unittests/test_slice_op.py

@@ -784,7 +784,7 @@ class TestInferShape(unittest.TestCase):
         self.assertEqual(x.shape, (3, -1, 5))
 
         out0 = paddle.slice(x, axes=[1], starts=[0], ends=[3])
-        self.assertEqual(out0.shape, (3, 3, 5))
+        self.assertEqual(out0.shape, (3, -1, 5))
 
     def test_axis_less_than_zero(self):
         # Using paddle.disable_static will make other unittests fail.