[llava] Read optimum-executorch pin from ci_commit_pins instead of hardcoding (#18457)

The `install_requirements.sh` script for the `llava` example was updated
to read the `optimum-executorch` version from the central pin file,
ensuring the installed version matches the project's pinned dependency.
Also updated then pin to latest commit on main
([`585799`](huggingface/optimum-executorch@585799c)).

Co-authored-by: Manuel Candales <42380156+manuelcandales@users.noreply.github.com>

Author: pytorchbot

.ci/docker/ci_commit_pins/optimum-executorch.txt

@@ -1 +1 @@
-a9592258daacad7423fd5f39aaa59c6e36471520
+585799cf7039d376d2ac4848b5ef0b501f60679e

examples/models/llava/install_requirements.sh

@@ -7,4 +7,6 @@
 
 set -x
 
-pip install git+https://github.com/huggingface/optimum-executorch.git@d4d3046738ca31b5542506aaa76a28d540600227
+EXECUTORCH_ROOT="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/../../.." && pwd)"
+OPTIMUM_ET_VERSION=$(cat "${EXECUTORCH_ROOT}/.ci/docker/ci_commit_pins/optimum-executorch.txt")
+pip install "git+https://github.com/huggingface/optimum-executorch.git@${OPTIMUM_ET_VERSION}"