Add an env PYTORCH_TEST_SKIP_CUDAGRAPH to skip all cuda graph-related unit tests (#103032)
Skip all cuda graph-related unit tests by setting env var `PYTORCH_TEST_SKIP_CUDAGRAPH=1`
This PR refactors the `TEST_CUDA` python variable in test_cuda.py into common_utils.py. This PR also creates a new python variable `TEST_CUDA_GRAPH` in common_utils.py, which has an env var switch to turn off all cuda graph-related tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103032
Approved by: https://github.com/malfet
