This make use of raft's slicing kernel within NN Descent build.
I found that my previous implementation was inefficient (merged in [this PR](#2345)).

### Improvements
Time to call NN Descent `build()` with `return_distances=True` before and after using this kernel
| Dataset | Before |After|
| ------------- | ------------- |---|
| mnist (60000, 784)  | 1550ms  | 1020ms|
| sift (1M, 128)  | 11342ms  |5546ms|
| gist (1M, 960)  | 13508ms |9278ms|

Authors:
  - Jinsol Park (https://github.com/jinsolp)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)
  - Micka (https://github.com/lowener)

URL: #2380

This PR is an amalgamation of the diff of 3 PRs in RAFT:

1. rapidsai/raft#2345
2. rapidsai/raft#2380
3. rapidsai/raft#2403

This PR also addresses part 1 and 2 of #419, closes #391 and makes CAGRA use the compiled headers of NN Descent, which seemed to have been a pending TODO https://github.com/rapidsai/cuvs/blob/009bb8de03ce9708d4d797166187250f77a59a36/cpp/src/neighbors/detail/cagra/cagra_build.cuh#L36-L37

Also, batch tests are disabled in this PR due to issue rapidsai/raft#2450. PR #424 will attempt to re-enable them.

Authors:
  - Divye Gala (https://github.com/divyegala)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #421