set small grf for dynamic cast

[jianyizh]

Previous solution for #1423 is reverted because other kernels like reduction may have performance regression. Now we set 128 grf only for loops kernel when dynamic cast.

[Copilot]

Pull Request Overview

Introduce an option to launch certain SYCL loop kernels with a reduced (128) GRF size using experimental SYCL properties, applied selectively to dynamic-cast loop paths to avoid prior regressions.

• Adds sycl_kernel_submit_small_grf with GRF size property and a distinct kernel name wrapper.
• Extends existing loop launch templates with a force_small_grf boolean template parameter and applies it (set to true) in specific kernel invocation paths.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/comm/SYCLHelpers.h	Adds small-GRF submission helper using experimental launch_config and grf_size property.
src/ATen/native/xpu/sycl/Loops.h	Extends kernel launch templates with force_small_grf flag and applies small-GRF launches in scalar/dynamic cast paths.

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