Ensure user sets backend to local w/ quantization

[Infernaught]

Previously: users could omit backend from their config when running LLM finetuning with quantization

Now: users are prompted to set backend to local explicitly when running LLM finetuning with quantization

[github-actions]

Unit Test Results

       6 files  ±       0         6 suites  ±0   1h 39m 15s ⏱️ + 21m 10s
2 812 tests  -        4  2 788 ✔️  -      15  21 💤 +  9  3 ❌ +2 
8 401 runs  +5 542  8 338 ✔️ +5 501  54 💤 +33  9 ❌ +8 

For more details on these failures, see this check.

Results for commit 0bc61a3. ± Comparison against base commit 3691dbe.

[justinxzhao]

The change LGTM. It looks like there's some failing test. @Infernaught can you take a look?

[arnavgarg1]

LGTM

[tgaddair]

This is very aggressive check that is already causing user-facing issues (#3529).

I understand the motivation for raising an error if the user selects quantization and the Ray backend, but that's not what this is doing. This is raising an error if the user doesn't EXPLCITLY select the local backend, which is quite extreme.

We should NEVER require user to specify the local backend explicitly if the local backend is going to be used anyway. Going to need to revert this and yank the v0.8.1 release.