Pull Request resolved: #20463

`aten.clone.default` is a pure flat copy on the buffer-only WebGPU backend, identical to `view_copy`: `clone_impl` reuses the existing `add_flat_copy` helper (`output[i] = input[i]`) and registers a handler under `aten.clone.default`. No new shader, generated WGSL header, or CMake source — it shares the `view_copy` flat-copy compute pipeline. Required for end-to-end Llama 3.2 1B (4-bit, KV cache): the exported model serializes 2 `aten.clone.default` ops into its runtime operator chain (the RoPE-frequency clones reused across all 16 transformer layers), so without a handler the partition graph-breaks at those nodes. Mirrors the Vulkan delegate, which registers the same op and routes a buffer clone to a flat view-copy.
ghstack-source-id: 397534700
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@diff-train-skip-merge

Differential Revision: [D109477717](https://our.internmc.facebook.com/intern/diff/D109477717/)

Pull Request resolved: #20464

Adds the WebGPU delegate handler for aten.index.Tensor, the 1D-self advanced-index
gather out[i] = self[index[i]] (output shape == index shape). This is the form the
VulkanPartitioner delegates -- it requires a 1D self and exactly one non-None index
(op_registry.py); 2D mask/freqs gathers stay on CPU. It mirrors the Vulkan delegate's
index_tensor op (IndexTensor.cpp + index_tensor_buffer.glsl) as a single compute
dispatch over the output elements, each reading the int32 index and gathering the
corresponding fp32 self element.

The op is composed as:
- index.wgsl: one workgroup-strided pass, out[i] = self[u32(index[i])], guarded by a
  numel bound; buffer-only, fp32 self/out, int32 index, 1D dispatch via the shared
  WebGPUUtils helpers (clamp workgroup size + 1D count).
- Index.cpp: validates the args (self/out tensors; indices ValueList with exactly one
  index tensor; fp32 self/out; int32 index; out numel == index numel), failing loud on
  any violation, then records the dispatch. row_width is dropped (always 1 for 1D self).
ghstack-source-id: 397756251
@exported-using-ghexport

@diff-train-skip-merge

Differential Revision: [D109478967](https://our.internmc.facebook.com/intern/diff/D109478967/)