Your sixth generation iPad Pro 12.9” was supplied in the box with a 20W USB-C Power Adapter. The iPad will charge at its maximum supported charging rate when using this Power Adapter. You can use a higher rated Power Adapter if you wish, but your iPad will not charge any faster.
Your iPad Pro (and many other models of iPad) does support “fast charging” - but only when using a USB PD (Power Delivery) compliant Power Adapter. USB PD negotiates the charging voltage with a similarly compliant device. When using a standard USB Power Adapter, that operates using 5V, the maximum charging rate of your iPad is current-limited to 2.4A (this being 12V at 5V).