[From pretrained] Allow download from subfolder inside model repo
# What does this PR do?

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Currently it is not possible for `transformers` to download a model that is located inside a subfolder of a repo.
E.g. for diffusion pipelines, a transformer model is often only one part of a pipeline of models so it makes a lot of sense to save checkpoints inside folders of model repos, see: https://huggingface.co/fusing/latent-diffusion-text2im-large/tree/main/bert

Similarly for Dalle-mini where one would have a Bart and a VQ-VAE model inside the same repo.

The PR would allow the user to do the following (which fails on master currently):

```py
from transformers import BertModel
BertModel.from_pretrained("fusing/latent-diffusion-text2im-large", revision="d5eab56", subfolder="bert")
```

**🚨🚨 IMPORTANT 🚨🚨**:

This PR adds subfolder loading and saving functionality for both sharded and non-sharded PyTorch checkpoints. It should also work when loading a model with `from_tf=True` or `from_flax=True` - however this is currently not tested. I would be great if such tests could be added in a follow-up PR.

Also cc @julien-c FYI
_The documentation is not available anymore as the PR was closed or merged._