[trainer] feat: Enabled fused adamw (#3692)

### What does this PR do?

Enable fused adamw which should be generally faster and more memory
efficient.
Also provide the config parameter to set eps.

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### Test

In our internal code base, we always set fused adamw to True and works
fine. However right now, I don't have the step time comparison with and
without it.
At the same time, I can push same change in RL code too and setting to
True should be generally beneficial.

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Author: puneeshkhanna

verl/trainer/fsdp_sft_trainer.py

@@ -322,6 +322,8 @@ def _build_model_optimizer(self):
             lr=self.config.optim.lr,
             betas=self.config.optim.betas,
             weight_decay=self.config.optim.weight_decay,
+            eps=self.config.optim.get("eps", 1e-08),
+            fused=True,
         )
 
         log_gpu_memory_usage("After initialize optimizer", logger=logger)