HfModelHandler: Call save_metadata after model is saved

[jambayk]

Describe your changes

• Previously, we save the metadata before calling save_pretrained on the modified PreTrained model object. This was done so that save_metadata doesn't override any files saved by save_pretrained.
• However, this leads to SameFileError if the model was loaded from a local path with trust_remote_code=True since it also tries to save the module files.
• To support this case as well as correctly update the auto_map in the config file to point to the saved files, we now call save_metadata after save_pretrained in `save_model_files. This is made possible with the following changes:
  • config is loaded from the save directory if already present
  • generation config and tokenizer are not saved if they already exist.
• test_hf_model is more comprehensive now with both local and hf-hub models.

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.
• Is this PR including examples changes? If yes, please remember to update example documentation in a follow-up PR.

(Optional) Issue link

Fixes #1442