Fix test_cast (#9951)

Author: anirudh2290

tests/python/unittest/test_operator.py

@@ -3229,8 +3229,8 @@ def test_cast():
             exe.arg_arrays[0][:] = X
             exe.forward(is_train=True)
             exe.backward(mx.nd.array(X, dtype=dsttype, ctx=default_context()))
-            assert_almost_equal(exe.outputs[0].asnumpy(), X.astype(srctype).astype(dsttype), rtol=1e-3)
-            assert_almost_equal(exe.grad_arrays[0].asnumpy(), X.astype(dsttype).astype(srctype), rtol=1e-3)
+            assert_almost_equal(exe.outputs[0].asnumpy(), X.astype(srctype).astype(dsttype), rtol=1e-3, atol=1e-5)
+            assert_almost_equal(exe.grad_arrays[0].asnumpy(), X.astype(dsttype).astype(srctype), rtol=1e-3, atol=1e-5)
 
 
 @with_seed()