Support Hugging Face data split code example

[ehsantn]

Changes included in this PR

Adds support for HuggingFace data split code examples that select things like "train" from a constant dictionary.

See here as an example (click on "use this dataset" then "pandas"): https://huggingface.co/datasets/openai/gsm8k

import pandas as pd

splits = {'train': 'main/train-00000-of-00001.parquet', 'test': 'main/test-00000-of-00001.parquet'}
df = pd.read_parquet("hf://datasets/openai/gsm8k/" + splits["train"])

A bunch of error checking tests needed to be updated since the tested patterns now work.

Testing strategy

Added a unit test.

User facing changes

The above code pattern works now.

Checklist

• Pipelines passed before requesting review. To run CI you must include [run CI] in your commit message.
• I am familiar with the Contributing Guide
• I have installed + ran pre-commit hooks.

[codecov]

Codecov Report

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Additional details and impacted files

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[scott-routledge2]

LGTM