mining is really just verifying transactions
Not correct.
Mining is a "proof of work", in the case of Bitcoin it's competing to be the first to find a hash that meets certain parameters (difficulty), for a block referencing the previous top one. Whether the new block has transactions in it or not, you get the same reward for being the first one to find and broadcast it.
Verifying is done by every node in the P2P network, both when deciding whether to relay candidate transactions, and when checking whether a new block's hash meets the mining requirements.
The Bitcoin blockchain has plenty of valid blocks with no transactions in them (part of a speculative mining strategy used by some to get the block reward faster than others).
The whole scheme works the same with any other kind of "proof of work", as long as the nodes relaying the new block can check whether the work happened or not (there are many ways in which that could be accomplished for AI training, the easiest of them by publishing the new model and having nodes check whether it meets some quality parameters).