enable use source code path build for sycl-tla

[jikunshang]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

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Purpose

enable use source code path build for sycl-tla

Test Plan

Test Result

(Optional) Documentation Update

BEFORE SUBMITTING, PLEASE READ https://docs.vllm.ai/en/latest/contributing (anything written below this line will be removed by GitHub Actions)

[Copilot]

Pull Request Overview

This PR enables building vllm-xpu-kernels with a custom cutlass-sycl source code path and updates the repository reference from cutlass-sycl to sycl-tla.

Key Changes:

• Adds support for specifying a custom CUTLASS source directory via the VLLM_CUTLASS_SRC_DIR environment variable
• Updates the Git repository URL from intel/cutlass-sycl to intel/sycl-tla
• Adds documentation for using a custom cutlass source code base

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
README.md	Adds documentation explaining how to use a custom cutlass-sycl code base via the VLLM_CUTLASS_SRC_DIR environment variable
CMakeLists.txt	Implements logic to use custom CUTLASS source directory when VLLM_CUTLASS_SRC_DIR is set, and updates the Git repository URL to sycl-tla

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[Copilot]

Changed 'step' to 'these steps' for grammatical correctness.

Suggested change

	if you want to use a custom cutlass-sycl code base, you can follow step:
	if you want to use a custom cutlass-sycl code base, you can follow these steps:

[Copilot]

Added space after '#' to follow Markdown comment convention: '# prepare cutlass source code'.

Suggested change

	#prepare cutlass source code
	# prepare cutlass source code

[Copilot]

Missing variable expansion syntax. Should be ${VLLM_CUTLASS_SRC_DIR} to properly check if the path is absolute.

Suggested change

	if(NOT IS_ABSOLUTE VLLM_CUTLASS_SRC_DIR)
	if(NOT IS_ABSOLUTE "${VLLM_CUTLASS_SRC_DIR}")