Data loader refactor (#2707)

* data loading refactor (wip)

* updates

* progress

* pytest

* pytest fix

* lint

* zero_first -> filelock, more simplifications

* small simplification

* import change

* nit

* lint

* simplify dedup

* couldnt resist

* review comments WIP

* continued wip

* minor changes

* fix; remove contrived test

* further refactor

* set default seed in pydantic config

* lint

* continued simplication

* lint

* renaming and nits

* filelock tests

* fix

* fix

* lint

* remove nullable arg

* remove unnecessary code

* moving dataset save fn to shared module

* remove debug print

* matching var naming

* fn name change

* coderabbit comments

* naming nit

* fix test

tests/e2e/multigpu/solo/test_grpo.py

@@ -105,7 +105,7 @@ def start_vllm(
                 print(f"{i}: VLLM server failed to start: {str(exc)}")
 
             # also check if the process.pid is still running
-            if not process.poll() is None:
+            if process.poll() is not None:
                 break
 
             time.sleep(period_seconds)