Verl's megatron core_r0.11.0 backend successfully tested with 3D parallelism with multiple bug fixed (#495)

This PR combines multiple modifications.

# QWen2.5 checkpoint saver bug fix

Thanks for the efforts @uygnef contributed to #368 , we use the new
saver for model loader and saver for 3D parallelism support.

# Megatron backend 3D-parallelism test benches

We modify the scripts in `examples/ppo_trainer` and `tests/e2e`, as well
as the CI workflows, all tested.

# Bug Fix for 3D-parallelism

Including configuration bugs as well as the module packing.

Original TP VocabParallelEntropy can lead to CUDA OOM, we refactor the
implementation with `torch.bmm`.

# Fully migration to Megatron Core

Now we only use Megatron core in verl, fully get rid of calling other
components. If they are in need, please integrate them into
`utils/megatron`.

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Co-authored-by: uygnef <admin@fengyu.org>