Updating DyNet configuration parameters

Is there a way to update the parameters after DyNet is initialized? For example, in a hyperparameter-tuning script, I'd like to change `weight_decay` for each new sample of hyperparameters.

I don't understand the difference between `DynetParams` and `dynet_config`. The [documentation](http://dynet.readthedocs.io/en/latest/python_ref.html) says I can use either, but `dynet_config` must be used before `import dynet`. However, it seems like I can't use `DynetParams` after `import dynet` either, because once I call `dyparams.init()` I get

WARNING: Attempting to initialize dynet twice. Ignoring duplicate initialization.