[WebGPU EP] Fix unnecessary CPU fallback and improve fallback diagnostic

[mingmingtasd]

Some nodes in MobileNet and the SD-Turbo pipeline were assigned to the CPU EP even though the WebGPU EP can run them, causing extra GPU<->CPU copies and, in the device-free compile-only flow (where CPU fallback is disabled), aborting session initialization outright. This adds the missing WebGPU kernel coverage and improves the diagnostics that explain why a node is not claimed by an EP. With these fixes all four SD-Turbo sessions (text encoder, UNet, VAE, safety checker) partition fully onto the WebGPU EP with zero CPU fallback.

Kernel coverage fixes, by model

MobileNet — AveragePool (pool.cc, webgpu_execution_provider.cc): register kernels for opsets 11-18, 19-21 and 22, replacing the single opset-11 registration. Root cause: MobileNet (opset 21) uses AveragePool at SinceVersion 19. ORT clamps an open-ended SinceVersion(11) registration to [11, 18] (not [11, INT_MAX]), so the opset-19 node was not claimed and fell back to the CPU EP; the CPU-only NchwcTransformer (Level3) then amplified the single pool into 3 CPU nodes.

SD-Turbo text encoder (CLIP) — Clip int32/uint32 (unary_elementwise_ops.cc, webgpu_execution_provider.cc): register int32 and uint32 Clip for opsets 11-11, 12-12 and 13. Root cause: the CLIP text position-id / index subgraph uses integer Clip, which was previously CPU-only.

SD-Turbo text encoder (CLIP) — Clip int64 (unary_elementwise_ops.cc, webgpu_execution_provider.cc): add a dedicated ClipInt64 kernel/shader for opsets 11-11, 12-12 and 13. Root cause: the WebNN-inserted index Clip before ArgMax (Inserted_Clip_951) is int64. The templated Clip supports only 4-byte element types (sizeof(T)==sizeof(float) static_assert), so int64 previously stayed on the CPU EP and, under compile-only (CPU fallback disabled), stopped the text encoder session from compiling. WebGPU has no native 64-bit integer type; the EP already stores int64 as vec2<u32> and operates on the truncated low 32 bits as i32 (shader_variable.cc GetByOffset/SetByOffset). Because Clip is monotonic, ClipInt64 clamps that i32 value (min/max saturated into i32 range) and sign-extends on write, consistent with the EP's existing int64 semantics.

SD-Turbo safety checker — Cast to uint8 (cast.cc, shader_variable.cc): allow the WebGPU Cast to produce uint8 output. Add uint8 to the Cast output (T2) type constraint, a to-uint8 shader expression, and a Uint8x4 packing path in SetByOffset (4 uint8 packed per u32, low element -> low byte -- the same byte layout already used for bool output). Root cause: WebNN has no bool type and represents a bool graph output as uint8, so the safety checker ends in bool->uint8 casts (Inserted_Cast_972, Inserted_Cast_974). uint8 was not a supported WebGPU Cast output type, so these terminal casts fell back to the CPU EP. Non-terminal bool->uint8->int32 cast chains can be collapsed upstream by CastChainElimination (a Level1 rewrite rule), but a terminal cast has no downstream cast to fold into and must be produced directly, hence this uint8 output support.

Diagnostics

These do not change placement; they make unexpected fallbacks diagnosable.

• execution_provider.cc (IExecutionProvider::GetCapability): log (VERBOSE) when an EP has no kernel for a still-unassigned node, including op type, opset and domain.
• kernel_lookup.h (LookUpKernel): capture and log (VERBOSE) the reason TryFindKernel failed (version vs. type-constraint mismatch, or no registration at all). The reason was previously built but discarded by the pointer-only return, leaving only an opaque "kernel not found". This identified Inserted_Clip_951 as int64 (not int32) and the safety checker fallbacks as uint8 casts.
• session_state.cc (verbose node-placement dump): include the op set version and the input/output types for each node, so a fallback caused by an unsupported element type is obvious from the placement log.

[azure-pipelines]

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