Use `config.num_channels` in CLIP-like modeling files
# What does this PR do?

`config.num_channels` is not used in some CLIP-like modeling files. Unlike previous PRs like #20596 or #20844, we use this attribute in the modeling files in this PR. 

The only breaking case is when a user previously set `config.num_channels=X` with `X !=3`, which is super unlikely IMO. (Even they did so, the actual Conv2D layer still uses `3` as it is hard-coded in the current `main` branch) 
_The documentation is not available anymore as the PR was closed or merged._