BufferList slice/negative-index support

[sudhansu-24]

Describe your changes

Add Python sequence protocol support to BufferList and clean up downstream usage.

BufferList ergonomics / minor performance cleanup

• Extend BufferList.getitem to support Python slicing (buffer_list[1:], buffer_list[-2:], step slices) and negative indices.

Downstream cleanup

• Remove redundant list(...) wrapping in BaseHiGraphModel.process_step by relying on the new slice behavior.

Tests

• Add unit tests for BufferList integer indexing, negative indexing, slicing semantics, and iteration.

Motivation / context
BufferList is used throughout the hierarchical graph models to store per-level tensors. Supporting slicing and negative indices makes hierarchical processing code more Pythonic and eliminates the need for explicit list() conversions, matching standard Python container expectations.

The create_graph.py visualization fix from the original PR has been dropped per @sadamov's guidance, that code path will be superseded by the weather-model-graphs migration (#83).

Dependencies
No new dependencies.

Issue Link

N/A (split from #471 — the visualization portion will be resolved by #83)

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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[sadamov]

@sudhansu-24 thanks, similar to other PRs about the create_graph script I am here to warn you that we plan to soon move to weather-model-graph #83 for graph creation. That repo has beautiful (and hopefully accurate ;)) plotting in place. unless you have a strong preference to get this fixed right now, I propose to close this PR and wait for #83, let me know what you think.

[sudhansu-24]

Hi @sadamov , thanks for the heads up!
I wasn't aware of the plan to move to weather-model-graphs (#83) that makes total sense.

I'm happy to drop the create_graph.py visualization fix since wmg already handles that properly. However, the BufferList slice/negative-index support (in utils.py) & coresponding cleanup in base_hi_graph_model.py are unrelated to create_graph would it make sense to split those into a separate PR, or would you prefer to close this entirely??

[sadamov]

@sudhansu-24 that makes sense, let's focus this one #472 on the bufferlist then

[sudhansu-24]

@sadamov Updated #472 to focus only on BufferList slice/negative-index support and the related unit tests. removed the create_graph.py visualization changes now, marked the PR as ready for review