Add CSR segment_softmax functional

[mnabian]

PhysicsNeMo Pull Request

Description

Adds segment_softmax, a backend-dispatched functional for numerically stable softmax over CSR/ragged segments.

This is intended as a low-level primitive for sparse/local attention over variable-size neighborhoods, such as anchor-to-point aggregation, graph attention, and ragged neighborhood attention without padding or PyG/torch_scatter.

weights = segment_softmax(logits, offsets)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
• An issue is linked to this pull request.
• If I am implementing a new model or modifying any existing model, I have followed the Models Implementation Coding Standards.

Dependencies

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