[NN module] speed up _load_from_state_dict (#85743)
Fixes #61398
The original implementation is very slow when the state_dict.keys() is long. This PR only passes relevant keys to the child module.
existing test passes: `pytest test/test_nn.py -k state_dict`
I couldn't figure out a good way to write a new test for this new behavior. Had a new snippet, but it will be flaky if integrated into the main CI because it's a timing based check.
But I can verify that the test took 30s to run, after this PR it only takes 0.5s.
```python
def test_load_state_dict_large(self):
# construct a module with 4 levels of module, 10 linear each, leads to 10k items in the dictionary
import copy
import time
base_module = nn.Linear(1,1)
model = base_module
for level in range(4):
model = nn.Sequential(*[copy.deepcopy(model) for _ in range(10)])
state_dict = model.state_dict()
self.assertEqual(len(state_dict), 20000)
st = time.time()
model.load_state_dict(state_dict, strict=True)
strict_load_time = time.time() - st
# it took 0.5 seconds to
self.assertLess(strict_load_time, 10)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85743
Approved by: https://github.com/albanD
