SYCL: using graphs is configurable by environment variable and compile option (#12371)

SYCL: using graphs is configurable by environment variable and compile option (#12371)

* alberto changes

* enable sycl graphs by env variable

* fixed compilation warnings in ggml-sycl.cpp

* renamed graph variables

* fix markdown in docs/backend/SYCL.md

Co-authored-by: Romain Biessy <redacted>
* fix markdown in docs/backend/SYCL.md again

* compiling graphs by default, renamed graph_enable to graph_disable

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Co-authored-by: Romain Biessy <redacted>