[SYCL]set context default value to avoid memory issue, update guide (#9476)
* set context default to avoid memory issue, update guide
* Update docs/backend/SYCL.md
Co-authored-by: Meng, Hengyu <hengyu.meng@intel.com>
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Co-authored-by: arthw <14088817+arthw@users.noreply.github.com>
Co-authored-by: Meng, Hengyu <hengyu.meng@intel.com>