Add function that submissions can call that can change the dropout value

[priyakasimbeg]

Our current API has 2 dropout related limitations:

1. Currently, in the external tuning ruleset we read the dropout value from the hparam config and pass it to the model initialization functions. In the self-tuning ruleset there exist no convenient way to specify the dropout value in the model initialization.
2. Furthermore, there is no way to change the dropout value during training.

Having a workload function to change the dropout value that submitters can call will remove both of these limitations.

[priyakasimbeg]

dedupe issue/753