[WebGPU] Support int8 data in Gather (#33790)
### Description
<!-- Describe your changes. -->
Adds int8 data support to the WebGPU `Gather` kernel.
- `int8` is added to the `T` type constraint of every WebGPU Gather
registration (opsets 1-10, 11-12, 13+).
- int8 uses the packed-byte path that bool and uint8 already use (four
1-byte elements per `u32`). Gather only moves data, so int8 is copied as
raw bytes. `unpack4xU8` reads each byte, and the `Int8x4` `SetByOffset`
keeps each lane's low byte. The two's-complement bit pattern, and so the
sign, is preserved.
- bool, uint8 and int64 behavior is unchanged.
Tests in `onnxruntime/test/providers/webgpu/tensor/gather_op_test.cc`
run only on the WebGPU EP with `kOrtSessionOptionsDisableCPUEPFallback`
set, and skip if the EP is unavailable:
- `Gather_int8_axis0_webgpu` / `Gather_int8_axis1_webgpu`: values
include -128, -1, 0 and 127. They use int32 and int64 indices, negative
indices, 2-D indices, and output sizes that do and don't fill the last
packed word.
- `Gather_int8_scalar_indices_webgpu`: 0-D indices, giving 0-D and 1-D
outputs.
- `Gather_int8_all_opsets_webgpu`: opsets 7, 11 and 13, one for each
registration.
- `Gather_int8_all_byte_values_webgpu`: all 256 int8 values, with 257
outputs that span two workgroups and end on a partial word.
- `Gather_int8_then_cast_to_float_webgpu`: Gather(int8) -> Cast(float)
graphs with runtime and initializer tables, 2-D and 0-D indices, and two
Gathers on one table. This mirrors the WebNN conformance graphs and
catches an int8 table that is read as unsigned.
The existing all-EP CPU tests `Gather_axis1_indices2d_int8` and
`Gather_axis1_neg_indices2d_int8` now also run on the WebGPU kernel.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
The new WPT page
[`webnn/conformance_tests/gather-cast-int8-table.https.any.js`](https://wpt.live/webnn/conformance_tests/gather-cast-int8-table.https.any.js)
fails 6 required subtests on the WebGPU EP. Each failing graph reads an
int8 table with `gather` (constant or runtime table, followed by `cast`
to float32 and sometimes `mul`/`reshape`, 0-D indices, or two gathers on
one table). The WebGPU Gather kernel didn't register int8:
```
ep.cc:96 Ep::GetCapabilityImpl webgpu kernel not found in registries for Op type: Gather
This session contains graph nodes that are assigned to the default CPU EP, but fallback to CPU EP has been explicitly disabled
```
WebNN disables CPU fallback, so these graphs failed to build. The
cast-then-gather variants already passed.
This is one of several WebGPU EP fixes for WebNN conformance.
**Validation**
- Built locally with the WebGPU EP. This commit was merged with main
`121e34d92b` together with a separate Relu int32 change.
- `onnxruntime_provider_test
--gtest_filter="WebGpuActivationTest.*:ActivationOpTest.Relu*:GatherOpTest.*:CastOpTest.*:GatherElementsOpTest.*:GatherNDOpTest.*"`:
**passed**, 268/268, 0 skipped. This includes all six new
`Gather_int8_*_webgpu` tests and the existing `Gather_axis1_*_int8`
tests, which now run on WebGPU.
- Negative check: removing the
`type_constraints.push_back(DataTypeImpl::GetTensorType<int8_t>());`
line makes all six `Gather_int8_*` tests **fail** at session init with
`fallback to CPU EP has been explicitly disabled`. This is the same
error the WPT page hits.
- `lintrunner -a` and `clang-format --dry-run -Werror` (20.1.8) on the
changed files: clean.
- WPT in the browser. Setup: plugin EP built from main `121e34d92b`,
this PR and the Relu int32 PR (local merge `5c68bbb9f9`); Edge Canary
159.0.4372.0; RTX Spark N1X / D3D12; ORT runtime 1.28.3.
- `webnn/conformance_tests/gather-cast-int8-table.https.any.js?gpu`:
**PASS 9/9**. Before this change, 6 required subtests failed with
"Failed to build graph".
- `webnn/conformance_tests/gather.https.any.js?gpu`: still **PASS
39/39**.
Co-authored-by: Aditya Rastogi <adityar@ntdev.microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
