Use uniform type names for init values throughout CUB

[gonidelis]

fixes #7974

enforces existing CUB naming convention:

• InitValueT: template parameters use PascalCase + T suffix (e.g. InputIteratorT, ReductionOpT, AccumT). These are part of the public/dispatch interface.
• init_value_t: local using aliases inside function bodies use snake_case + _t suffix.

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• cub/cub/device/dispatch/dispatch_streaming_reduce.cuh

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• cub/cub/device/dispatch/dispatch_streaming_reduce.cuh

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Summary

Standardizes the naming of "initial-value" types across CUB to follow the project convention:

• Template parameters: PascalCase with trailing T → InitValueT
• Local using/type-aliases: snake_case with _t suffix → init_value_t

This is a mechanical rename/propagation: identifiers and local aliases were changed and the new names were propagated through dispatch templates, kernel templates, internal helpers, benchmarks, and tests. No algorithmic, behavioral, or functional changes were introduced.

Scope of changes

• Files touched: ~28 files including benchmarks, core device headers, dispatch plumbing, kernel templates, and tests.
• Key areas:
  • Benchmarks: renamed local aliases (init_t → init_value_t) and updated usages.
  • Device APIs/headers: internal template parameter renames (InitT → InitValueT) and local alias renames.
  • Dispatch plumbing: propagated InitValueT through Dispatch* templates, defaults, and stored init members/parameters.
  • Kernels: kernel template parameters and init parameter types renamed to InitValueT; added/adjusted unwrap/finalize helpers for empty_problem_init_t.
  • Tests: Catch2 tests and reference helpers updated to use init_value_t / InitValueT.

Impact assessment

• Public API: No semantic changes to runtime behavior. The edits are identifier renames (template parameter/alias names) and do not alter algorithmic behavior or dispatch logic.
• Source-level compatibility: Code that relied on the literal identifier name InitT in manual instantiations or documentation may need small updates; typical user code should be unaffected.
• Tests/benchmarks: Updated accordingly; no test behavior changes expected.

Notes for reviewers

• Verify all instances of InitT/init_t (and similar variants) were consistently replaced with InitValueT/init_value_t.
• Check accumulator-default computations and all places where AccumT is derived from the init-type default to ensure they now derive from InitValueT.
• Confirm empty_problem_init_t handling and unwrap/finalize helpers were updated to accept the new InitValueT-based overloads where appropriate.
• Watch for missed occurrences in documentation comments or rare instantiation sites that might still reference the old names.

suggestion: ## Walkthrough

This PR uniformly renames initial-value type identifiers from InitT/init_t to InitValueT/init_value_t across dispatch templates, kernel signatures, device API callsites, benchmarks, and tests.

suggestion: ## Changes

Initial value type naming consolidation

Layer / File(s)	Summary
Dispatch type contracts cub/cub/device/dispatch/dispatch_reduce.cuh, cub/cub/device/dispatch/dispatch_reduce_deterministic.cuh, cub/cub/device/dispatch/dispatch_reduce_nondeterministic.cuh, cub/cub/device/dispatch/dispatch_segmented_reduce.cuh, cub/cub/device/dispatch/dispatch_streaming_reduce.cuh	Template parameter InitT renamed to InitValueT throughout dispatch signatures, accumulator selection, and detail::reduce/detail::segmented_reduce dispatch entrypoints.
Kernel parameter adaptation cub/cub/device/dispatch/kernels/kernel_reduce.cuh, cub/cub/device/dispatch/kernels/kernel_reduce_deterministic.cuh, cub/cub/device/dispatch/kernels/kernel_segmented_reduce.cuh	Kernel template parameters and signatures renamed from InitT to InitValueT; helpers unwrap_empty_problem_init and finalize_and_store_aggregate updated to handle empty_problem_init_t<InitValueT>.
Device public APIs (internal aliases) cub/cub/device/device_reduce.cuh, cub/cub/device/device_scan.cuh, cub/cub/device/device_segmented_reduce.cuh	Internal aliases renamed to init_value_t/InitValueT in convenience overloads (Sum, Min, Max, ArgMin, ArgMax) and dispatch call sites; public API signatures unchanged.
Benchmarks cub/benchmarks/bench/reduce/*, cub/benchmarks/bench/scan/exclusive/*, cub/benchmarks/bench/segmented_reduce/*, cub/benchmarks/bench/transform_reduce/sum.cu	Local aliases and tuned policy accumulator template arguments updated to use init_value_t and initializers constructed as init_value_t{}.
Test helpers and tests cub/test/*	Reference helper templates, test callsites, FutureValue/test buffers, and environment kernel-instantiation checks updated to use InitValueT/init_value_t and to pass init_value_t{} into device APIs.

suggestion: ## Assessment against linked issues

Objective	Addressed	Explanation
Consolidate initial value type naming to a single uniform name [#7974]	✅
Use InitValueT/init_value_t consistently across device headers [#7974]	✅
Update dispatch, kernel, and device API implementations to use unified naming [#7974]	✅

suggestion: ## Suggested reviewers

• bernhardmgruber
• oleksandr-pavlyk
• srinivasyadav18

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