[DETR] Add num_channels attribute
# What does this PR do?

This PR adds a num_channels attribute to DetrConfig, making it possible to use the model on greyscale images (i.e. num_channels = 1).

Fixes #14875
_The documentation is not available anymore as the PR was closed or merged._<|||||>In my previous experience with DETR I also have faced the issues with Feature extractor, does it support 1 chanel input now? (I had to do something like that: https://github.com/huggingface/transformers/pull/14933/files#diff-826e0e0303aa4a12e527884e4dc548fe8debb137905260e42ec8b4331c9c2c63R594) @NielsRogge 
