suggest a numerical limit of 50MB for determining @slow
This is a follow up to https://github.com/huggingface/transformers/issues/7250 which adds a guideline to when making a test `@slow` based on the download requirements if any.

The suggested value is >50MB, and we can adjust it later if it's too large or small.

Fixes: https://github.com/huggingface/transformers/issues/7250

@LysandreJik, @sgugger 