feat(build): select operator implementations from ops.json

[voltjia]

Summary

• Extends INFINI_OPS_OPS so it accepts a user-owned ops.json that selects exact implementation slots per operator.
• Uses one normalized selection for wrapper generation, linked dependency resolution, and generated Torch operators.
• Preserves the existing comma-separated operator allowlist and legacy external header-list JSON format.
• Removes the need for a separate INFINI_OPS_LINKED_OPS configuration surface.

Example:

{
  "add": {"implementations": [0]},
  "argmax": {"implementations": [8]},
  "top_k_top_p_sampling_from_logits": {"implementations": [16]}
}

Motivation

Operator names alone cannot express builds that need one implementation while excluding another provider of the same operator. Selecting by the existing implementation slot keeps native, generated Torch, and linked providers under the same configuration model and avoids resolving or linking unselected external dependencies.

Compatibility

• -DINFINI_OPS_OPS=add,gemm retains its current operator-level behavior.
• -DINFINI_OPS_OPS=/absolute/path/ops.json enables slot selection.
• A repository-root user ops.json remains supported when the CMake option is unset.
• The repository does not ship a default ops.json; the file is a consumer-owned build input.
• Slot arrays are sets, not priority lists. Runtime default selection remains the first active slot.

Validation

• Ruff format/check passed.
• Focused selector, wrapper, Torch codegen, and linked resolver tests: 93 passed.
• Concrete CMake configurations covered inline precedence, JSON slot filtering, missing external providers, and linked slot resolution.
• Final selector commit is based directly on master and does not contain feat(nvidia): add top-k top-p sampling provider #922.

Type of Change

• feat
• build / CMake
• tests
• docs

Notes

The FlashInfer sampling provider is intentionally a separate stacked change. This PR only establishes the generic implementation-selection mechanism.

[voltjia]

Superseded by #931 after renaming the head branch to match the feat(build) PR title. GitHub automatically closed this PR during the branch rename; the replacement points to the same 4ba65ce commit.