Delete `state_dict` to release memory as early as possible
# What does this PR do?

Fix #18782.

Note that this is not a real memory issue. A call to `gc.collect()` at the end of `from_pretrained()` works well too.

However this PR finds and simply `del state_dict` at the end of `_load_state_dict_into_model()`, and `GC` is able to perform housekeeping on its own at a earlier time.
_The documentation is not available anymore as the PR was closed or merged._<|||||>The change regarding having a new argument `state_dict` in the nested function `load` is to pass black check, otherwise we get 

```bash
src/transformers/modeling_utils.py:422:17: F821 undefined name 'state_dict'
```
with the new line `del state_dict`. (It's quite strange though)<|||||>Ready for review.<|||||>The failing test is `test_encodings_from_xnli_dataset` which is irrelevant to this PR.