Mark gradient_test as flaky (#1067)

The test in `gradient_test.py` fails randomly in CI about 50% of the
time. This then wastes maintainer time looking at the logs, re-running
the checks on PRs, and babysitting things. It's especially annoying when
it happens during merge queue events.

This PR simply marks that test as flaky using Bazel's feature for that
purpose. That should in turn cause Bazel to retry the failed test.

Author: mhucka

tensorflow_quantum/python/differentiators/BUILD

@@ -134,6 +134,7 @@ py_test(
     name = "gradient_test",
     timeout = "eternal",
     srcs = ["gradient_test.py"],
+    flaky = True,
     python_version = "PY3",
     deps = [
         ":adjoint",