Migrate all DataLoader::Load callsites (#4291)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4291

Starts to completely deprecate DataLoader::Load by converting existing callsites to DataLoader::load, and making DataLoader::load pure virtual.

Searched for all relevant callsites by doing the following:
- Grep for all instances of `core/data_loader.h>` to find all direct imports of DataLoader, finding "Load(" in the file
- Grep for all instances of `executorch\/.*_data_loader.h>` to find all imports of subclasses derived from DataLoader, finding "Load(" in the file
- Grep for all instances of "->Load(" in files with "executorch" in the path
- Grep for all isntances of "loader->Load(" and "loader_->Load(" in the entire codebase

Reviewed By: dbort

Differential Revision: D59767505

fbshipit-source-id: d8a6d998f957dcba291815312f8bde54f84c3100