Implement load_into for the shared_ptr data loader

[JacobSzwejbka]

🚀 The feature, motivation and pitch

load_into is used if mutable state has a meaningful initial value. This primarily happens with on-device training but could also happen if some model had read before write state that we serialized an initial value for.

For this task we need to implement this function https://github.com/pytorch/executorch/blob/main/runtime/core/data_loader.h#L111 in https://github.com/pytorch/executorch/blob/main/extension/data_loader/shared_ptr_data_loader.h

example of implementation in filedataloader https://github.com/pytorch/executorch/blob/main/extension/data_loader/file_data_loader.h#L75

Then add a corresponding test to https://github.com/pytorch/executorch/blob/main/extension/data_loader/test/shared_ptr_data_loader_test.cpp

Alternatives

No response

Additional context

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RFC (Optional)

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cc @larryliu0820 @lucylq

[Tanish2101]

I would like to work on this.

@JacobSzwejbka