It's difficult to buff out plastic without causing more damage. You might try using a liquid auto polish. Generally a scratch remover compound will damage plastic. Auto parts shops sell all types of polish, so look around.
If you have a Plastics shop in your area you could take your iPad Pro down to them to have a look at the marks as a good plastics shop usually has a product for removing scratches in plastics, and they tend to fill in the low scratch areas and then get "polished" out.
The opposite method is using a grinding compound as suggested, and one of the finest available that can sometimes work is silver or brass/copper polish. Silvo and Brasso are trade names that can work.
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