Add safeguards for CUDA kernel load in Deformable DETR
# What does this PR do?

Transformers has become unusable on GPU when doing some imports since:
- deformable DETR compiles some custom CUDA kernels at init
- this require ninja which is not in the dependencies

This PR adds some additional checks before compiling the CUDA kernels, and also adds a warning instead of a hard error when that load fails.
_The documentation is not available anymore as the PR was closed or merged._