feat: add special_shifted_chebyshev_polynomial_u base

[voltjia]

Summary

• Add the hand-written InfiniOps base class for special_shifted_chebyshev_polynomial_u in src/base/special_shifted_chebyshev_polynomial_u.h.
• Let the torch code generator reuse src/base/special_shifted_chebyshev_polynomial_u.h instead of emitting generated/base/special_shifted_chebyshev_polynomial_u.h.
• Apply the base class member-spacing convention required by scripts/check_conventions.py.

Motivation

This PR is part of the feat/torch-codegen base-header migration. The generated SpecialShiftedChebyshevPolynomialU base declaration is moved into src/base so code generation can reuse a reviewed hand-written header.

N/A: no linked issue.

Type of Change

• feat - new feature / new operator / new platform
• fix - bug fix
• perf - performance improvement (no behavioral change)
• refactor - code restructuring without behavior change
• test - adding or fixing tests only
• docs - documentation only
• build / ci - build system or CI configuration
• chore - tooling, formatting, or other non-code changes
• Breaking change (requires a ! in the Conventional Commits prefix or a BREAKING CHANGE: footer)

Platforms Affected

• CPU (WITH_CPU)
• NVIDIA (WITH_NVIDIA)
• Iluvatar (WITH_ILUVATAR)
• MetaX (WITH_METAX)
• Cambricon (WITH_CAMBRICON)
• Moore (WITH_MOORE)
• Ascend (WITH_ASCEND)
• PyTorch C++ bindings (WITH_TORCH)
• Build system / CMake / CI
• Python bindings / user-facing API

Test Results on Supported Platforms

Platform	Built	pytest Result	Notes / Hardware
NVIDIA	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.
Iluvatar	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.
MetaX	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.
Cambricon	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.
Moore	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.
Ascend	N/A	Not run	Not required for this non-master feat/torch-codegen base-header PR; no runtime implementation is added.

Full `pytest` output (optional)

N/A: pytest was intentionally not run because this PR targets `feat/torch-codegen`, not `master`, and only adds a reusable base header declaration.

Benchmark / Performance Impact

N/A. This PR only adds a base operator declaration for torch codegen reuse and does not add a runtime implementation.

Notes for Reviewers

• This PR targets feat/torch-codegen, not master.
• The branch diff against feat/torch-codegen contains only src/base/special_shifted_chebyshev_polynomial_u.h.
• Original branch validation reported clang-format 21 passing on src/base/special_shifted_chebyshev_polynomial_u.h; the follow-up formatting commit applies the class member spacing required by scripts/check_conventions.py.

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Checklist

Title, Branch, and Commits

• PR title follows Conventional Commits (e.g. feat(nvidia): …, fix(cuda/gemm): …).
• N/A: this automated batch uses existing codex/add-special_shifted_chebyshev_polynomial_u-base PR branches targeting feat/torch-codegen; branch renaming is intentionally out of scope.
• Each commit message follows Conventional Commits.
• N/A: this batch intentionally keeps the base-header addition and convention-formatting follow-up as two meaningful, squashable commits.
• N/A: this PR is based on feat/torch-codegen, not master; no master rebase is required for this integration target.
• No fixup! / squash! / wip commits remain.

Scope and Design

• Changes are minimal - nothing unrelated to the stated motivation was added (CONTRIBUTING.md §Code/General).
• No dead code, commented-out blocks, debug prints, printf/std::cout/print(...) left behind, or TODO without an owner and issue link.
• No unrelated formatting churn that would obscure the diff.
• Public API changes are intentional and limited to the SpecialShiftedChebyshevPolynomialU base operator declaration used by torch codegen.

General Code Hygiene (applies to all languages)

• The code is self-explanatory; comments were added only where the why is non-obvious (CONTRIBUTING.md §Code/General).
• Every modified or added file ends with a single trailing newline (CONTRIBUTING.md §Code/General).
• No trailing whitespace, tab/space mixing, or stray BOMs.
• Identifiers in comments and error messages are wrapped in backticks (e.g. the `seqlens_k` tensor) (CONTRIBUTING.md §Code/General).
• All comments and error messages are in English (CONTRIBUTING.md §Code/General).
• Comments and error messages are complete sentences - capitalized first letter, terminal punctuation - unless the language/framework convention says otherwise (CONTRIBUTING.md §Code/General; §Python).

C++ Specific (if C++ files changed)

• Code follows the Google C++ Style Guide strictly.
• clang-format (version 21, per .github/workflows/clang-format.yml) has been run against all modified .h, .cc, .cuh, and .mlu files; the diff is clean.
• N/A: clang-tidy was not run because this PR only adds a base declaration header for feat/torch-codegen; no runtime implementation is added.
• Operator parameter order is inputs first, outputs last; attributes are between inputs and outputs; naming follows PyTorch → ONNX → CUDA API precedence (CONTRIBUTING.md §C++).
• N/A: this base declaration does not add C++ error paths or exceptions.
• N/A: this base declaration does not add error or warning messages.
• N/A: this base declaration does not add kernel files.
• N/A: this base declaration does not add kernel launchers.
• Constructor initializer list order matches member declaration order (CONTRIBUTING.md §C++).
• Exactly one blank line between classes, between classes and functions, and between functions (CONTRIBUTING.md §C++).
• Exactly one blank line between members (functions and variables) within a class (CONTRIBUTING.md §C++).
• Exactly one blank line before and after the contents of a namespace (CONTRIBUTING.md §C++).
• N/A: this PR adds only src/base/special_shifted_chebyshev_polynomial_u.h for torch codegen reuse; platform implementations are out of scope.
• No raw new/delete; RAII / smart pointers / existing allocators are used.

Python Specific (if Python files changed)

N/A: no Python files changed.

Testing

• N/A: platform pytest was intentionally not run because this PR targets feat/torch-codegen, not master, and only adds a reusable base header declaration.
• N/A: the table above records the reason platform testing was skipped.
• N/A: no runtime functionality was added, so no new tests/ coverage is required.
• N/A: no new pytest parameterization was added.
• N/A: no Payload-returning test was added.
• N/A: no dtype / device parameterization was added.
• N/A: no flaky test was added.
• N/A: this is not a runtime bug fix.

Build, CI, and Tooling

• N/A: full platform builds were not run because this PR targets feat/torch-codegen, not master, and only adds a reusable base header declaration.
• N/A: compile_commands.json behavior was not changed.
• N/A: no new backend or device was added.
• N/A: CUDA-like backend mutual exclusion was not changed.
• Existing CI formatting expectations are preserved; original validation reported clang-format 21 passing on src/base/special_shifted_chebyshev_polynomial_u.h.
• N/A: no new runtime dependency was added.

Documentation

• N/A: README.md, CONTRIBUTING.md, and developer workflow are unchanged.
• N/A: SpecialShiftedChebyshevPolynomialU is an internal base declaration for torch codegen reuse; no user-facing documentation is required.
• N/A: no user-visible breaking change.

Security and Safety

• No secrets, access tokens, internal URLs, customer data, or personal hardware identifiers have been committed.
• N/A: no third-party code was added.
• N/A: no unsafe pointer arithmetic, uninitialized reads, or missing bounds checks were introduced.