[megatron] fix: Remove Deprecated Megatron Optimizer Args (verl-project#4396)

### What does this PR do?

This PR removes the deprecated arguments during Megatron optimizer
building for compatibility with the latest Megatron, see
[https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/core/optimizer/__init__.py#L442](https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/core/optimizer/__init__.py#L442).
These arguments are never used by verl so they can be safely removed.
This solves the following exception with the latest Megatron:

```
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 456, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
           ^^^^^^^^^^^^^^^^^^^^^
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/single_controller/ray/base.py", line 825, in func
    return getattr(self.worker_dict[key], name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/single_controller/base/decorator.py", line 451, in inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/utils/transferqueue_utils.py", line 187, in dummy_inner
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/workers/megatron_workers.py", line 573, in init_model
    ) = self._build_model_optimizer(
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/workers/megatron_workers.py", line 464, in _build_model_optimizer
    actor_optimizer = get_megatron_optimizer(model=actor_module, config=optim_config_megatron)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/verl/verl/utils/megatron/optimizer.py", line 71, in get_megatron_optimizer
    return get_megatron_optimizer_native(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: get_megatron_optimizer() got an unexpected keyword argument 'no_weight_decay_cond'
```

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Author: DaizeDong

verl/utils/megatron/optimizer.py

@@ -63,17 +63,11 @@ def init_megatron_optim_config(optim_config: dict, fp16: bool = False) -> Optimi
 def get_megatron_optimizer(
     model,
     config: OptimizerConfig,
-    no_weight_decay_cond=None,
-    scale_lr_cond=None,
-    lr_mult=1.0,
 ):
     # Base optimizer.
     return get_megatron_optimizer_native(
         config=config,
         model_chunks=model,
-        no_weight_decay_cond=no_weight_decay_cond,
-        scale_lr_cond=scale_lr_cond,
-        lr_mult=lr_mult,
     )