Migrate to RAPIDS-Triton

[wphicks]

This PR moves the FIL backend codebase over to the new RAPIDS-Triton library. It attempts to do so with minimal changes in functionality, with the following exceptions:

• A new configuration parameter (transfer_threshold) has been introduced, which allows users to conditionally move data to the GPU for processing only if the batch size exceeds some threshold
• The end-to-end custom build method has been completely removed. It was largely deprecated in 21.10 and would have required significant effort to migrate to the new codebase

Besides that, other changes (including explicit testing of categorical support and other potential code reorganization suggested by the RAPIDS Triton template) have been reserved for subsequent PRs.

Resolve #144 Resolve #141 Resolve #125 Resolve #60 Resolve #14 Resolve #34 Resolve #135

[wphicks]

The Triton 21.10 container has now been released, so I just updated the base build container.