mark `test_save_load_fast_init_from_base` as `is_flaky`
# What does this PR do?

Mark `test_save_load_fast_init_from_base` as `is_flaky`.
 - This test is known to be flaky, see #19849
 - The level of flakiness seems to get higher after #20042
 - **ran 5 times, and all passed**.
 - **TODO**: check why #20042 makes this test more flaky.
_The documentation is not available anymore as the PR was closed or merged._<|||||>I guess the best first step is to run it 1000 times (in a loop) and see how many times it fails (before and after your PR 20042).

Sometimes it is difficult to get reproducible failure if it is flaky.<|||||>As I commented before, this probably comes from models having weights initialized outside of the `_init_weights` function. A way to debug would be to drop somewhere which weight was randomly dropped when the test has failed (if it's printed for instance, we can look for it in the logs)