implement cos gradient as a function op (#13227)
### Description
Implemented gradient of cos as per the function below.
### Motivation and Context
Cos gradient required for [huggingface's diffusers
library](https://github.com/huggingface/diffusers)
### Testing
built ORT from source: `./build.sh --config RelWithDebInfo
--enable_training --use_cuda --cuda_home /usr/local/cuda --cudnn_home
/usr/local/cuda --build_wheel --parallel --skip_tests`
tested CosGrad implementation: `cd build/Linux/RelWithDebInfo/ &&
./onnxruntime_test_all --gtest_filter=GradientCheckerTest.CosGrad`
Co-authored-by: Prathik Rao <prathikrao@microsoft.com>
