Update `max_diff` in `test_save_load_fast_init_to_base`
# What does this PR do?

This test seems flaky. After looking a bit deeper, I am not sure if we should expect to get the same (or very close) weights with/without `_fast_init` for the deleted key.
https://github.com/huggingface/transformers/blob/9ecb13d63a9524478656b2233e6fb4e9f15d3fbf/tests/test_modeling_common.py#L343
https://github.com/huggingface/transformers/blob/9ecb13d63a9524478656b2233e6fb4e9f15d3fbf/tests/test_modeling_common.py#L400

My intuition is that **the values for that deleted key could be different with 2 different init. methods.**

I also change the way of `max_diff` being calculated.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you @patrickvonplaten! Although I still have some doubt:

Assumption we have:
> now this initialization should be the same for both the fast and slow init method

What we do
> _init_weights weights is overwritten by a deterministic self._mock_init_weights

But `_mock_init_weights` is only defined in the testing module, and basically it just does `data.fill_(3)`.
So the assumption is only True in our own testing (which uses `_mock_init_weights`). This won't be the case when we want to load the model outside the testing. So I am not very sure the purpose of this testing.

But good for me if we don't want to touch it. We probably need to add some common about the flakyness for some tests though.
 <|||||>@sgugger Could you check if the change in the way `max_diff` being calculated worth the merge 🙏 ? Thanks