[Modeling] Reduce runtime when loading missing keys
Author: kylesayrs
Pull request: yes

## Purpose ##
* Reduce runtime when loading missing keys with `low_cpu_mem_usage`
  * This is particularly helpful when loading large models

## Changes ##
* Do not eagerly convert all keys into a list. Instead, check membership directly to preserve O(1) lookup within the for loop

## Testing ##
* This script can be used to load a large model without a state dict for testing
  * Without these changes, loading deepseek_v3 takes 229.1s
  * With these changes, loading deepseek_v3 takes 64.5s (~4x speedup)

<details><summary>load_without_weights_for_testing.py</summary>

```python3
import os
import torch
import tempfile
import contextlib
from huggingface_hub import snapshot_download
from transformers.utils import SAFE_WEIGHTS_INDEX_NAME, WEIGHTS_INDEX_NAME
from safetensors.torch import save_file

import time
from transformers import AutoModelForCausalLM, AutoConfig, PreTrainedModel
from accelerate import init_empty_weights

## Define utils

@contextlib.contextmanager
def skip_weights_download(model_class: PreTrainedModel = AutoModelForCausalLM):
    """
    Context manager under which models are initialized without having to download
    the model weight files

    :param model_class: class to patch, `AutoModelForCausalLM`
    """
    original_fn = model_class.from_pretrained
    weights_files = [
        "*.bin", "*.safetensors", "*.pth", SAFE_WEIGHTS_INDEX_NAME, WEIGHTS_INDEX_NAME
    ]

    @classmethod
    def patched(cls, *args, **kwargs):
        nonlocal tmp_dir

        # intercept model stub
        model_stub = args[0] if args else kwargs.pop("pretrained_model_name_or_path")

        # download files into tmp dir
        os.makedirs(tmp_dir, exist_ok=True)
        snapshot_download(
            repo_id=model_stub,
            local_dir=tmp_dir,
            ignore_patterns=weights_files
        )

        # make an empty weights file to avoid errors
        weights_file_path = os.path.join(tmp_dir, "model.safetensors")
        save_file({}, weights_file_path, metadata={"format": "pt"})

        # load from tmp dir
        return original_fn(tmp_dir, **kwargs)
    
    with tempfile.TemporaryDirectory() as tmp_dir:
        model_class.from_pretrained = patched
        yield
        model_class.from_pretrained = original_fn


@contextlib.contextmanager
def skip_weights_initialize():
    def skip(tensor: torch.Tensor, *args, **kwargs) -> torch.Tensor:
        return tensor

    kaiming_restore = torch.nn.init.kaiming_uniform_
    uniform_restore = torch.nn.init.uniform
    normal_restore = torch.nn.init.normal_

    t_uniform_restore = torch.Tensor.uniform_
    t_normal_restore = torch.Tensor.normal_

    torch.nn.init.kaiming_uniform_ = skip
    torch.nn.init.uniform_ = skip
    torch.nn.init.normal_ = skip

    torch.Tensor.uniform_ = skip
    torch.Tensor.normal_ = skip
    try:
        yield
    finally:
        torch.nn.init.kaiming_uniform_ = kaiming_restore
        torch.nn.init.uniform_ = uniform_restore
        torch.nn.init.normal_ = normal_restore

        torch.Tensor.uniform_ = t_uniform_restore
        torch.Tensor.normal_ = t_normal_restore


## Load model
model_name = "deepseek-ai/DeepSeek-V3"

# needed for deekseek
config = AutoConfig.from_pretrained(model_name, trust_remote_code=True)
del config.quantization_config

# load model
start = time.time()
with skip_weights_download(), skip_weights_initialize():
    model = AutoModelForCausalLM.from_pretrained(
        model_name,
        config=config,
        device_map="auto",
        trust_remote_code=True,
    )
    print(f"Loaded model in {time.time() - start:.1f}s")
```
</details>

Comments:
@Rocketknight1 All good on my end, thanks!

Merged, and thank you for the improvement @kylesayrs!