Fixed examples/walking.py and updated README run_mode command

[johnshizhu]

fixed walking.py example and README run_mode command.

1. Updated PPOTask _apply_gradients_with_clipping function to _apply_gradients only,
2. clipping added to walking.py optax.chain() with clip_by_global_norm,
3. added global_grad_clip field to HumanoidWAlkingTaskConfig
4. Update README command to run_mode=view

*** Left grad_metrics as an empty xax.FrozenDict in the _apply_gradients function

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