Add selector.is_et_kernel_key_selected (#103184)
Summary:
This API is used by the gen_executorch.py to check whether a kernel with specified kernel key is used or not.
Test Plan:
```
buck test xplat/caffe2/tools:test_torchgen_executorch
buck run fbcode//executorch/codegen/tools:test_gen_oplist_real_model
```
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103184
Approved by: https://github.com/larryliu0820
