Add remote checkpoint support to ray tune post search evaluation

[amholler]

Recently Ludwig was updated to perform model evaluation after the Ray Tune search was complete,
for any trials that were terminated before their model evaluation step was performed. This update did not
handle the case where the checkpoint was remote, e.g., when output_directory was set to an s3 bucket.
This change set adds that handling. Will create a cross-port of this change set for master.

[github-actions]

Unit Test Results

       3 files   -     3         3 suites   - 3   1h 45m 56s ⏱️ - 1h 2m 13s
1 135 tests  -   81  1 119 ✔️  -   73  16 💤  -   8  0 ❌ ±0 
3 405 runs   - 243  3 357 ✔️  - 219  48 💤  - 24  0 ❌ ±0 

Results for commit accd0d9. ± Comparison against base commit 5192a2a.