Training demo (#5445)

Summary:
Allows the XOR model training demo to be runnable in OSS. Will follow up with a documentation PR about training and how to run this demo.

Im sure my cmakelist.txt changes have issues so if anyone sees ways to improve them please let me know.

Only hack I had to do was the optimizer was calling an ET op directly. I don't think we have enabled this in OSS yet so I will follow up with larryliu0820 when hes back and in the meantime open up an issue.

Repro of demo:

Pull Request resolved: https://github.com/pytorch/executorch/pull/5445

Test Plan:
python3 extension/training/examples/XOR/export_model.py --outdir /tmp/xor

rm -rf cmake-out

mkdir cmake-out

cmake \
    -DCMAKE_INSTALL_PREFIX=cmake-out \
    -DCMAKE_BUILD_TYPE=Release \
    -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
    -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
    -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON \
    -DEXECUTORCH_BUILD_EXTENSION_TRAINING=ON \
    -DEXECUTORCH_ENABLE_LOGGING=ON \
    -DPYTHON_EXECUTABLE=python \
    -Bcmake-out .

cmake --build cmake-out -j9 --target install --config Release

./cmake-out/extension/training/train_xor --model_path=/tmp/xor/xor.pte

Reviewed By: dvorjackz

Differential Revision: D62905840

Pulled By: JacobSzwejbka

fbshipit-source-id: 622e68637ee7a0bb1b323e777d60e9516be115cd