fix(cudnn): remove duplicate ConvBwdData match arm in backend graph builder

[CharryWu]

Summary

Removes a duplicate unreachable match arm in the cuDNN backend graph builder.

Problem

In GraphBuilder::build(), the operations iterator map had two identical arms for Operation::ConvBwdData. The second arm was unreachable dead code and would generate a compiler warning under strict linting.

Change

Remove the duplicate arm. All seven operation variants (ConvBwdData, ConvBwdFilter, ConvFwd, MatMul, Pointwise, Reduction) are now each matched exactly once.

Testing

• cudnn crate compiles cleanly (Compiling cudnn v0.1.0 passes)
• One-line change, no logic affected

Flow Diagrams

crates/cudnn/src/lib.rs
└── mod backend  (src/backend/mod.rs)
    ├── descriptor.rs    ─── Descriptor (Rc<Inner> wrapping cudnnBackendDescriptor_t)
    ├── operation.rs     ─── Operation enum (the 6 DNN operation types)
    ├── graph.rs         ─── GraphBuilder → Graph  ◄── PR #372 fix here
    ├── engine_heuristic.rs  ─── HeuristicMode (picks best engine for a graph)
    ├── engine.rs / engine_cfg.rs  ─── Engine configuration
    ├── execution_plan.rs    ─── ExecutionPlan (final executable plan)
    ├── tensor.rs            ─── Tensor descriptors
    ├── conv_fwd.rs          ─── ConvFwd operation builder
    ├── conv_bwd_data.rs     ─── ConvBwdData operation builder
    ├── conv_bwd_filter.rs   ─── ConvBwdFilter operation builder
    ├── matmul.rs            ─── MatMul operation builder
    ├── pointwise.rs         ─── Pointwise operation builder
    └── reduction.rs         ─── Reduction operation builder

Data flow for cuDNN backend graph execution:

 ┌─────────────────────────────────────────────────────────────────────┐
 │                     User-facing build pipeline                      │
 │                                                                     │
 │  Tensor ──┐                                                         │
 │  Tensor ──┼──► Operation (ConvFwd / ConvBwdData / MatMul / ...)     │
 │  ConvCfg ─┘         │                                               │
 │                     ...  (repeat for each op)                       │
 │                      │                                              │
 │                      ▼                                              │
 │              Vec<Operation>                                         │
 │                      │                                              │
 │  CudnnContext ───┐   │                                              │
 │                  ▼   ▼                                              │
 │              GraphBuilder                                           │
 │                  │                                                   │
 │                  │  .build()                                         │
 │                  ▼                                                   │
 │  ┌───────────────────────────────┐                                  │
 │  │  Graph                        │                                  │
 │  │  ├─ descriptor (finalized)    │  Wraps CUDNN_BACKEND_            │
 │  │  ├─ context                   │  OPERATIONGRAPH_DESCRIPTOR       │
 │  │  └─ operations                │                                  │
 │  └───────────────┬───────────────┘                                  │
 │                  │                                                   │
 │                  ▼                                                   │
 │          EngineHeuristic  (picks optimal algorithm)                 │
 │                  │                                                   │
 │                  ▼                                                   │
 │          EngineCfg  (configures chosen engine)                      │
 │                  │                                                   │
 │                  ▼                                                   │
 │          ExecutionPlan  (ready to execute on GPU)                   │
 └─────────────────────────────────────────────────────────────────────┘