[DeepSpeed] add weight_mapping to _load_state_dict_into_zero3_model
Author: kashif
Pull request: yes

# What does this PR do?

This pull request introduces enhancements to the DeepSpeed integration for model loading, specifically improving how weight conversions (such as renaming and merging/splitting of weights) are handled when loading state dictionaries into models using DeepSpeed ZeRO-3. The changes add a new utility for applying weight conversion logic before loading, and update the model loading pipeline to use this utility when needed.

Key changes:

### Weight Conversion Utilities

* Added a new function `_apply_weight_conversions_to_state_dict` in `deepspeed.py` to handle weight renaming and merging/splitting operations on state dicts before loading them into a model. This function supports both simple renaming and more complex conversions using `WeightConverter` and `WeightRenaming` objects.

### DeepSpeed Model Loading Logic

* Updated `_load_state_dict_into_zero3_model` to accept an optional `weight_mapping` argument. If provided, it uses the new `_apply_weight_conversions_to_state_dict` function to preprocess the state dict before loading. It also stores the applied weight conversions on the model for later reference.
* Modified the call to `_load_state_dict_into_zero3_model` in `_load_pretrained_model` (in `modeling_utils.py`) to pass the `weight_mapping` argument, ensuring that weight conversions are applied during DeepSpeed ZeRO-3 model loading.

Fixes #43257


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_43303). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

View the CircleCI Test Summary for this PR:

https://huggingface.co/spaces/transformers-community/circle-ci-viz?pr=43303&sha=8b03dd

i'll use the new `LoadStateDictConfig`