Pin transformers version in phi example

[jambayk]

Describe your changes

The latest version of transformers (>= 4.35.0) is not compatible with the model. PRs: huggingface/transformers#27020, huggingface/transformers#27073 change the expected signature of _set_gradient_checkpointing which now doesn't match the model's https://huggingface.co/microsoft/phi-1_5/blob/main/modeling_mixformer_sequential.py#L802

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• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Format your code by running pre-commit run --all-files
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.

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