…olcengine#1770)

### Checklist Before Starting

- [x] Search for similar PR(s).

### What does this PR do?

entropy_coeff shall be set carefully during RL. When enabled,
inappropriate coefficient may case training to collapse. You can see
more empirical experiments from Skywork Open Reasoner 1 Technical Report
(https://arxiv.org/pdf/2505.22312).

In this PR, the default value of entropy_coeff is set to 0. This is a
breaking change that may affect your experiment, although majority of
verl example scripts set it to 0 manually already.

We let most example script just pick up the default value of 0 for
entropy_coeff. For a few documentation page where the reference model
performance and commands are provided, we modify the doc so that the
experiment result is consistent with the config setup.

### Usage Example

To enable entropy loss coefficient, use 
```bash
actor_rollout_ref.actor.entropy_coeff=0.001 # or other values
```

### Test

> For changes that can not be tested by CI (e.g., algorithm
implementation, new model support), validate by experiment(s) and show
results like training curve plots, evaluatuion results, etc.

### Additional Info.

- **Issue Number**: Fixes issue # or discussion # if any.
- **Training**: [Note which backend this PR will affect: FSDP, Megatron,
both, or none]
- **Inference**: [Note which backend this PR will affect: vLLM, SGLang,
both, or none]

### Checklist Before Submitting

- [x] Read the [Contribute
Guide](https://github.com/volcengine/verl?tab=readme-ov-file#contribution-guide).
- [ ] Apply [pre-commit
checks](https://github.com/volcengine/verl?tab=readme-ov-file#code-linting-and-formatting).
- [x] Add `[BREAKING]` to the PR title if it breaks any API.
- [x] Update the documentation about your changes in the
[docs](https://github.com/volcengine/verl/tree/main/docs).
- [ ] Add CI test(s) if necessary.

…olcengine#1770)

### Checklist Before Starting

- [x] Search for similar PR(s).

### What does this PR do?

entropy_coeff shall be set carefully during RL. When enabled,
inappropriate coefficient may case training to collapse. You can see
more empirical experiments from Skywork Open Reasoner 1 Technical Report
(https://arxiv.org/pdf/2505.22312).

In this PR, the default value of entropy_coeff is set to 0. This is a
breaking change that may affect your experiment, although majority of
verl example scripts set it to 0 manually already.

We let most example script just pick up the default value of 0 for
entropy_coeff. For a few documentation page where the reference model
performance and commands are provided, we modify the doc so that the
experiment result is consistent with the config setup.

### Usage Example

To enable entropy loss coefficient, use 
```bash
actor_rollout_ref.actor.entropy_coeff=0.001 # or other values
```

### Test

> For changes that can not be tested by CI (e.g., algorithm
implementation, new model support), validate by experiment(s) and show
results like training curve plots, evaluatuion results, etc.

### Additional Info.

- **Issue Number**: Fixes issue # or discussion # if any.
- **Training**: [Note which backend this PR will affect: FSDP, Megatron,
both, or none]
- **Inference**: [Note which backend this PR will affect: vLLM, SGLang,
both, or none]

### Checklist Before Submitting

- [x] Read the [Contribute
Guide](https://github.com/volcengine/verl?tab=readme-ov-file#contribution-guide).
- [ ] Apply [pre-commit
checks](https://github.com/volcengine/verl?tab=readme-ov-file#code-linting-and-formatting).
- [x] Add `[BREAKING]` to the PR title if it breaks any API.
- [x] Update the documentation about your changes in the
[docs](https://github.com/volcengine/verl/tree/main/docs).
- [ ] Add CI test(s) if necessary.