A very nice visual layout is kept current at: http://iossupportmatrix.com/iOS Support Matrix
The top row has RAM size below the horizontal line showing the late 2016 and early 2017 models have 2, 3, or 4 GB of RAM across the board.
From Wikipedia:
- 1st generation: 256 MB
- iPad 2 and iPad Mini: 512 MB
- 3rd generation, 4th generation, iPad Air, iPad Mini 2 and iPad Mini 3: 1 GB
- iPad Air 2 and iPad Mini 4: 2 GB
- iPad Pro 10.5 and 12.9-inch: 4 GB
- iPad Pro 9.7-inch: 2 GB
