normalize keys_to_ignore
as discussed at https://github.com/huggingface/transformers/issues/16719#issuecomment-1156599137 this PR normalizes `_keys_to_ignore_on*` to not backslash on dot, unless it's an actual regex with regex patterns - this is just for consistency and easier troubleshooting. As sometimes it's unclear if `\` is needed or not when different modeling files use different styles.

@sgugger 

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